diff --git a/lib/codegen/fromcto/avro/avrovisitor.js b/lib/codegen/fromcto/avro/avrovisitor.js
index 107a3835..36cafffc 100644
--- a/lib/codegen/fromcto/avro/avrovisitor.js
+++ b/lib/codegen/fromcto/avro/avrovisitor.js
@@ -48,7 +48,7 @@ class AvroVisitor {
         }  else if (thing.isMapDeclaration?.()) {
             return;
         } else if (thing.isTypeScalar?.()) {
-            return this.visitField(thing.getScalarField(), parameters);
+            return this.visitField(thing.getScalarField(), parameters, thing.isOptional());
         } else if (thing.isField?.()) {
             return this.visitField(thing, parameters);
         } else if (thing.isRelationship?.()) {
@@ -167,17 +167,19 @@ class AvroVisitor {
      * Visitor design pattern
      * @param {Field} field - the object being visited
      * @param {Object} parameters  - the parameter
+     * @param {boolean} [isOptional] - whether the property is optional, defaults to `field.isOptional()`.
+     * Passed explicitly for scalar-typed properties, because `getScalarField()` does not copy `optional`.
      * @return {Object} the result of visiting or null
      * @private
      */
-    visitField(field, parameters) {
+    visitField(field, parameters, isOptional = field.isOptional()) {
         let avroType = this.toAvroType(field.getType());
 
         if (field.isArray()) {
             avroType = `array<${avroType}>`;
         }
 
-        if (field.isOptional()) {
+        if (isOptional) {
             avroType = `union { null, ${avroType} }`;
         }
 
diff --git a/lib/codegen/fromcto/graphql/graphqlvisitor.js b/lib/codegen/fromcto/graphql/graphqlvisitor.js
index 45fd6ceb..e1cef4ad 100644
--- a/lib/codegen/fromcto/graphql/graphqlvisitor.js
+++ b/lib/codegen/fromcto/graphql/graphqlvisitor.js
@@ -49,7 +49,7 @@ class GraphQLVisitor {
         } else if (thing.isModelFile?.()) {
             return this.visitModelFile(thing, parameters);
         } else if (thing.isTypeScalar?.()) {
-            return this.visitField(thing.getScalarField(), parameters);
+            return this.visitField(thing.getScalarField(), parameters, thing.isOptional());
         } else if (thing.isField?.()) {
             return this.visitField(thing, parameters);
         } else if (thing.isMapDeclaration?.()) {
@@ -140,17 +140,19 @@ class GraphQLVisitor {
     * Visitor design pattern
     * @param {Field} field - the object being visited
     * @param {Object} parameters  - the parameter
+    * @param {boolean} [isOptional] - whether the property is optional, defaults to `field.isOptional()`.
+    * Passed explicitly for scalar-typed properties, because `getScalarField()` does not copy `optional`.
     * @return {Object} the result of visiting or null
     * @private
     */
-    visitField(field, parameters) {
+    visitField(field, parameters, isOptional = field.isOptional()) {
         let type = this.toGraphQLType( this.namespaces ? field.getFullyQualifiedTypeName() : field.getType() );
 
         if(field.isArray()) {
             type = `[${type}]`;
         }
 
-        if(!field.isOptional()) {
+        if(!isOptional) {
             type = `${type}!`;
         }
 
diff --git a/lib/codegen/fromcto/protobuf/protobufvisitor.js b/lib/codegen/fromcto/protobuf/protobufvisitor.js
index 9cde4e0a..31abe0b4 100644
--- a/lib/codegen/fromcto/protobuf/protobufvisitor.js
+++ b/lib/codegen/fromcto/protobuf/protobufvisitor.js
@@ -41,14 +41,16 @@ class ProtobufVisitor {
     /**
      * Transform a Concerto meta property into a Proto3 field rule.
      * @param {Object} field - the Concerto meta property
+     * @param {boolean} [isOptional] - whether the property is optional, defaults to `field.isOptional()`.
+     * Callers pass it explicitly for scalar-typed properties, because `getScalarField()` does not copy `optional`.
      * @return {Object} the Proto3 field rule
      * @public
      */
-    concertoToProto3FieldRule(field) {
+    concertoToProto3FieldRule(field, isOptional = field.isOptional()) {
         // An array (repeated) in Proto3 is implicitly optional.
         if (field.isArray()) {
             return 'repeated';
-        } else if (field.isOptional()) {
+        } else if (isOptional) {
             return 'optional';
         } else {
             return null;
@@ -178,7 +180,7 @@ class ProtobufVisitor {
         } else if (thing.isClassDeclaration?.()) {
             return this.visitClassDeclaration(thing, parameters);
         } else if (thing.isTypeScalar?.()) {
-            return this.visitField(thing.getScalarField(), parameters);
+            return this.visitField(thing.getScalarField(), parameters, thing.isOptional());
         } else if (thing.isField?.()) {
             return this.visitField(thing, parameters);
         } else if (thing.isRelationship?.()) {
@@ -388,12 +390,13 @@ class ProtobufVisitor {
      * Visitor design pattern
      * @param {Object} field - the object being visited
      * @param {Object} parameters - the parameter
+     * @param {boolean} [isOptional] - whether the property is optional, defaults to `field.isOptional()`
      * @private
      */
-    visitField(field, parameters) {
+    visitField(field, parameters, isOptional = field.isOptional()) {
         debug('entering visitField', field.getName());
 
-        const preposition = this.concertoToProto3FieldRule(field);
+        const preposition = this.concertoToProto3FieldRule(field, isOptional);
 
         if (ModelUtil.isMap(field)) {
             const mapDeclaration = field.getModelFile().getType(field.getType());
diff --git a/lib/codegen/fromcto/rust/rustvisitor.js b/lib/codegen/fromcto/rust/rustvisitor.js
index a356bf05..4fd1fd93 100644
--- a/lib/codegen/fromcto/rust/rustvisitor.js
+++ b/lib/codegen/fromcto/rust/rustvisitor.js
@@ -190,7 +190,7 @@ class RustVisitor {
         } else if (thing.isMapDeclaration?.()) {
             return this.visitMapDeclaration(thing, parameters);
         } else if (thing.isTypeScalar?.()) {
-            return this.visitField(thing.getScalarField(), parameters);
+            return this.visitField(thing.getScalarField(), parameters, thing.isOptional());
         } else if (thing.isField?.()) {
             return this.visitField(thing, parameters);
         } else if (thing.isRelationship?.()) {
@@ -399,10 +399,12 @@ class RustVisitor {
      * Visitor design pattern
      * @param {Field} field - the object being visited
      * @param {Object} parameters - the parameter
+     * @param {boolean} [isOptional] - whether the property is optional, defaults to `field.isOptional?.()`.
+     * Passed explicitly for scalar-typed properties, because `getScalarField()` does not copy `optional`.
      * @return {Object} the result of visiting or null
      * @private
      */
-    visitField(field, parameters) {
+    visitField(field, parameters, isOptional = field.isOptional?.()) {
         // Check for decorators
         const decorators = field.getDecorators?.() || [];
 
@@ -511,7 +513,7 @@ class RustVisitor {
 
             // Determine HashMap type
             let hashMapType = `HashMap<${rustKeyType}, ${rustValueType}>`;
-            if (field.isOptional?.()) {
+            if (isOptional) {
                 hashMapType = this.wrapAsOption(hashMapType);
             }
             // Add description for HashMap fields
@@ -522,7 +524,7 @@ class RustVisitor {
             parameters.fileWriter.writeLine(2, `rename = "${field.name}",`);
 
             // Add skip_serializing_if for optional HashMap fields
-            if (field.isOptional?.()) {
+            if (isOptional) {
                 parameters.fileWriter.writeLine(
                     2,
                     'skip_serializing_if = "Option::is_none",'
@@ -535,7 +537,7 @@ class RustVisitor {
 
             if (isKeyDateTime && isValueDateTime) {
                 // Both key and value are DateTime
-                if (field.isOptional?.()) {
+                if (isOptional) {
                     parameters.fileWriter.writeLine(
                         2,
                         'serialize_with = "serialize_hashmap_datetime_both_option",'
@@ -556,7 +558,7 @@ class RustVisitor {
                 }
             } else if (isKeyDateTime) {
                 // Only key is DateTime
-                if (field.isOptional?.()) {
+                if (isOptional) {
                     parameters.fileWriter.writeLine(
                         2,
                         'serialize_with = "serialize_hashmap_datetime_key_option",'
@@ -577,7 +579,7 @@ class RustVisitor {
                 }
             } else if (isValueDateTime) {
                 // Only value is DateTime
-                if (field.isOptional?.()) {
+                if (isOptional) {
                     parameters.fileWriter.writeLine(
                         2,
                         'serialize_with = "serialize_hashmap_datetime_value_option",'
@@ -616,7 +618,7 @@ class RustVisitor {
 
         parameters.fileWriter.writeLine(1, '#[serde(');
         parameters.fileWriter.writeLine(2, `rename = "${field.name}",`);
-        if (field.isOptional?.()) {
+        if (isOptional) {
             parameters.fileWriter.writeLine(
                 2,
                 'skip_serializing_if = "Option::is_none",'
@@ -624,7 +626,7 @@ class RustVisitor {
             type = this.wrapAsOption(type);
         }
         if (this.isDateField(field.type)) {
-            if (field.isArray?.() && field.isOptional?.()) {
+            if (field.isArray?.() && isOptional) {
                 // Option<Vec<DateTime<Utc>>>
                 parameters.fileWriter.writeLine(
                     2,
@@ -644,7 +646,7 @@ class RustVisitor {
                     2,
                     'deserialize_with = "deserialize_datetime_array",'
                 );
-            } else if (field.isOptional?.()) {
+            } else if (isOptional) {
                 // Option<DateTime<Utc>>
                 parameters.fileWriter.writeLine(
                     2,
diff --git a/test/codegen/fromcto/avro/avrovisitor.js b/test/codegen/fromcto/avro/avrovisitor.js
index a841e726..e9bb0c71 100644
--- a/test/codegen/fromcto/avro/avrovisitor.js
+++ b/test/codegen/fromcto/avro/avrovisitor.js
@@ -39,6 +39,18 @@ concept Person identified by email {
 }
 `;
 
+const MODEL_OPTIONAL_SCALAR = `
+namespace test@1.0.0
+
+scalar SSN extends String
+
+concept Person {
+  o SSN ssn
+  o SSN taxId optional
+  o String name optional
+}
+`;
+
 describe('Avro', function () {
 
     describe('samples', () => {
@@ -76,4 +88,19 @@ describe('Avro', function () {
             }).should.throw('Unrecognised type: string, value: \'Something of unrecognised type\'');
         });
     });
+
+    describe('optional scalar properties', () => {
+        it('should generate an optional scalar property as a nullable union', () => {
+            const modelManager = new ModelManager();
+            modelManager.addCTOModel(MODEL_OPTIONAL_SCALAR);
+            const visitor = new AvroVisitor();
+            const fileWriter = new InMemoryWriter();
+            modelManager.accept(visitor, { fileWriter });
+            const file = fileWriter.getFilesInMemory().get('test@1.0.0.avdl');
+            file.should.match(/^\s*string ssn;$/m);
+            file.should.match(/^\s*union \{ null, string \} taxId;$/m);
+            // The same as an optional property declared with the primitive type
+            file.should.match(/^\s*union \{ null, string \} name;$/m);
+        });
+    });
 });
diff --git a/test/codegen/fromcto/graphql/graphqlvisitor.js b/test/codegen/fromcto/graphql/graphqlvisitor.js
index fab34b23..8f10f1c1 100644
--- a/test/codegen/fromcto/graphql/graphqlvisitor.js
+++ b/test/codegen/fromcto/graphql/graphqlvisitor.js
@@ -29,6 +29,7 @@ const Field = require('@accordproject/concerto-core').Field;
 const RelationshipDeclaration = require('@accordproject/concerto-core').RelationshipDeclaration;
 const FileWriter = require('@accordproject/concerto-util').FileWriter;
 const { ModelUtil } = require('@accordproject/concerto-core');
+const { InMemoryWriter } = require('@accordproject/concerto-util');
 
 let sandbox = sinon.createSandbox();
 
@@ -56,6 +57,18 @@ asset Vehicle {
 }
 `;
 
+const MODEL_OPTIONAL_SCALAR = `
+namespace test@1.0.0
+
+scalar SSN extends String
+
+concept Person {
+  o SSN ssn
+  o SSN taxId optional
+  o String name optional
+}
+`;
+
 describe('GraphQLVisitor', function () {
     let graphQLVisitor;
     let mockFileWriter;
@@ -424,4 +437,18 @@ ref: String
 arrayRef: String) on OBJECT | FIELD_DEFINITION`]);
         });
     });
+
+    describe('optional scalar properties', () => {
+        it('should not mark an optional scalar property as non-null', () => {
+            const modelManager = new ModelManager();
+            modelManager.addCTOModel(MODEL_OPTIONAL_SCALAR);
+            const fileWriter = new InMemoryWriter();
+            modelManager.accept(new GraphQLVisitor(), { fileWriter });
+            const file = fileWriter.getFilesInMemory().get('model.gql');
+            file.should.match(/^\s*ssn: String!$/m);
+            file.should.match(/^\s*taxId: String$/m);
+            // The same as an optional property declared with the primitive type
+            file.should.match(/^\s*name: String$/m);
+        });
+    });
 });
\ No newline at end of file
diff --git a/test/codegen/fromcto/protobuf/data/org.accordproject.protocol.v1_0_0-expected.proto b/test/codegen/fromcto/protobuf/data/org.accordproject.protocol.v1_0_0-expected.proto
index 7039a281..055da9cb 100644
--- a/test/codegen/fromcto/protobuf/data/org.accordproject.protocol.v1_0_0-expected.proto
+++ b/test/codegen/fromcto/protobuf/data/org.accordproject.protocol.v1_0_0-expected.proto
@@ -48,12 +48,12 @@ message Function {
   repeated string emittedTypes = 2;
   string name = 3;
   string requestType = 4;
-  string responseType = 5;
+  optional string responseType = 5;
 }
 
 message Logic {
   repeated Function functions = 1;
-  string stateType = 2;
+  optional string stateType = 2;
 }
 
 message Template {
@@ -89,7 +89,7 @@ message Agreement {
   Metadata metadata = 7;
   repeated string references = 8;
   repeated Signature signatures = 9;
-  string state = 10;
+  optional string state = 10;
   string template = 11;
 }
 
@@ -133,7 +133,7 @@ message _Subclasses_of_class_ConversionOptions {
 }
 
 message PdfConversionOptions {
-  string styles = 1;
+  optional string styles = 1;
 }
 
 enum FeatureType {
@@ -160,7 +160,7 @@ message TriggerResponse {
   optional string errorDetails = 1;
   optional string errorMessage = 2;
   bool isError = 3;
-  string result = 4;
+  optional string result = 4;
 }
 
 enum AgreementStatusType {
diff --git a/test/codegen/fromcto/protobuf/protobufvisitor.js b/test/codegen/fromcto/protobuf/protobufvisitor.js
index 077956df..983340f4 100644
--- a/test/codegen/fromcto/protobuf/protobufvisitor.js
+++ b/test/codegen/fromcto/protobuf/protobufvisitor.js
@@ -42,6 +42,18 @@ const ModelUtil = require('@accordproject/concerto-core').ModelUtil;
 let sandbox = sinon.createSandbox();
 
 
+const MODEL_OPTIONAL_SCALAR = `
+namespace test@1.0.0
+
+scalar SSN extends String
+
+concept Person {
+  o SSN ssn
+  o SSN taxId optional
+  o String name optional
+}
+`;
+
 describe('ProtobufVisitor', function () {
     let protobufVisitor;
     let mockFileWriter;
@@ -774,4 +786,18 @@ describe('ProtobufVisitor', function () {
             );
         });
     });
+
+    describe('optional scalar properties', () => {
+        it('should generate an optional scalar property with the optional keyword', () => {
+            const modelManager = new ModelManager();
+            modelManager.addCTOModel(MODEL_OPTIONAL_SCALAR);
+            const fileWriter = new InMemoryWriter();
+            modelManager.accept(new ProtobufVisitor(), { fileWriter });
+            const file = fileWriter.getFilesInMemory().get('test.v1_0_0.proto');
+            file.should.match(/^\s*string ssn = \d+;$/m);
+            file.should.match(/^\s*optional string taxId = \d+;$/m);
+            // The same as an optional property declared with the primitive type
+            file.should.match(/^\s*optional string name = \d+;$/m);
+        });
+    });
 });
diff --git a/test/codegen/fromcto/rust/rustvisitor.js b/test/codegen/fromcto/rust/rustvisitor.js
index 313856c3..b8aadd68 100644
--- a/test/codegen/fromcto/rust/rustvisitor.js
+++ b/test/codegen/fromcto/rust/rustvisitor.js
@@ -33,9 +33,22 @@ const ModelManager = require('@accordproject/concerto-core').ModelManager;
 const RelationshipDeclaration =
     require('@accordproject/concerto-core').RelationshipDeclaration;
 const FileWriter = require('@accordproject/concerto-util').FileWriter;
+const { InMemoryWriter } = require('@accordproject/concerto-util');
 
 let sandbox = sinon.createSandbox();
 
+const MODEL_OPTIONAL_SCALAR = `
+namespace test@1.0.0
+
+scalar SSN extends String
+
+concept Person {
+  o SSN ssn
+  o SSN taxId optional
+  o String name optional
+}
+`;
+
 describe('RustVisitor', function () {
     let rustVisitor;
     let mockFileWriter;
@@ -1809,4 +1822,18 @@ describe('RustVisitor', function () {
             ).calledOnce.should.be.ok;
         });
     });
+
+    describe('optional scalar properties', () => {
+        it('should generate an optional scalar property as an Option', () => {
+            const modelManager = new ModelManager();
+            modelManager.addCTOModel(MODEL_OPTIONAL_SCALAR);
+            const fileWriter = new InMemoryWriter();
+            modelManager.accept(new RustVisitor(), { fileWriter });
+            const file = fileWriter.getFilesInMemory().get('test_1_0_0.rs');
+            file.should.match(/pub ssn: String,/);
+            file.should.match(/rename = "taxId",\s+skip_serializing_if = "Option::is_none",\s+\)\]\s+pub tax_id: Option<String>,/);
+            // The same as an optional property declared with the primitive type
+            file.should.match(/rename = "name",\s+skip_serializing_if = "Option::is_none",\s+\)\]\s+pub name: Option<String>,/);
+        });
+    });
 });
diff --git a/types/lib/codegen/fromcto/avro/avrovisitor.d.ts b/types/lib/codegen/fromcto/avro/avrovisitor.d.ts
index 9f982dae..1903dbd8 100644
--- a/types/lib/codegen/fromcto/avro/avrovisitor.d.ts
+++ b/types/lib/codegen/fromcto/avro/avrovisitor.d.ts
@@ -57,6 +57,8 @@ declare class AvroVisitor {
      * Visitor design pattern
      * @param {Field} field - the object being visited
      * @param {Object} parameters  - the parameter
+     * @param {boolean} [isOptional] - whether the property is optional, defaults to `field.isOptional()`.
+     * Passed explicitly for scalar-typed properties, because `getScalarField()` does not copy `optional`.
      * @return {Object} the result of visiting or null
      * @private
      */
diff --git a/types/lib/codegen/fromcto/graphql/graphqlvisitor.d.ts b/types/lib/codegen/fromcto/graphql/graphqlvisitor.d.ts
index 6deecc6c..5eaca423 100644
--- a/types/lib/codegen/fromcto/graphql/graphqlvisitor.d.ts
+++ b/types/lib/codegen/fromcto/graphql/graphqlvisitor.d.ts
@@ -52,6 +52,8 @@ declare class GraphQLVisitor {
     * Visitor design pattern
     * @param {Field} field - the object being visited
     * @param {Object} parameters  - the parameter
+    * @param {boolean} [isOptional] - whether the property is optional, defaults to `field.isOptional()`.
+    * Passed explicitly for scalar-typed properties, because `getScalarField()` does not copy `optional`.
     * @return {Object} the result of visiting or null
     * @private
     */
diff --git a/types/lib/codegen/fromcto/protobuf/protobufvisitor.d.ts b/types/lib/codegen/fromcto/protobuf/protobufvisitor.d.ts
index b0afbff9..3fd8f057 100644
--- a/types/lib/codegen/fromcto/protobuf/protobufvisitor.d.ts
+++ b/types/lib/codegen/fromcto/protobuf/protobufvisitor.d.ts
@@ -17,10 +17,12 @@ declare class ProtobufVisitor {
     /**
      * Transform a Concerto meta property into a Proto3 field rule.
      * @param {Object} field - the Concerto meta property
+     * @param {boolean} [isOptional] - whether the property is optional, defaults to `field.isOptional()`.
+     * Callers pass it explicitly for scalar-typed properties, because `getScalarField()` does not copy `optional`.
      * @return {Object} the Proto3 field rule
      * @public
      */
-    public concertoToProto3FieldRule(field: any): any;
+    public concertoToProto3FieldRule(field: any, isOptional?: boolean): any;
     /**
      * Transform a Concerto primitive type into a Proto3 one.
      * @param {Object} type - the Concerto primitive type
@@ -128,6 +130,7 @@ declare class ProtobufVisitor {
      * Visitor design pattern
      * @param {Object} field - the object being visited
      * @param {Object} parameters - the parameter
+     * @param {boolean} [isOptional] - whether the property is optional, defaults to `field.isOptional()`
      * @private
      */
     private visitField;
diff --git a/types/lib/codegen/fromcto/rust/rustvisitor.d.ts b/types/lib/codegen/fromcto/rust/rustvisitor.d.ts
index c8e1ff0f..3c16f321 100644
--- a/types/lib/codegen/fromcto/rust/rustvisitor.d.ts
+++ b/types/lib/codegen/fromcto/rust/rustvisitor.d.ts
@@ -59,6 +59,8 @@ declare class RustVisitor {
      * Visitor design pattern
      * @param {Field} field - the object being visited
      * @param {Object} parameters - the parameter
+     * @param {boolean} [isOptional] - whether the property is optional, defaults to `field.isOptional?.()`.
+     * Passed explicitly for scalar-typed properties, because `getScalarField()` does not copy `optional`.
      * @return {Object} the result of visiting or null
      * @private
      */