diff --git a/src/lib/psbAdditionalInfoLength.test.ts b/src/lib/psbAdditionalInfoLength.test.ts
new file mode 100644
index 0000000..a45c5f9
--- /dev/null
+++ b/src/lib/psbAdditionalInfoLength.test.ts
@@ -0,0 +1,39 @@
+import { describe, expect, it } from "vitest";
+import {
+  readAdditionalInfoLength,
+  usesEightBytePsbAdditionalInfoLength,
+} from "./psbAdditionalInfoLength";
+
+describe("PSB additional layer information lengths", () => {
+  it("keeps normal tagged blocks at 4-byte lengths in PSB", () => {
+    const buffer = new ArrayBuffer(8);
+    const view = new DataView(buffer);
+    view.setUint32(0, 0x00000012, false);
+    view.setUint32(4, 0xdeadbeef, false);
+
+    expect(readAdditionalInfoLength(view, 0, true, "luni"))
+      .toEqual({ length: 0x12, bytesRead: 4 });
+  });
+
+  it("uses 8-byte lengths for the PSB keys defined by Adobe", () => {
+    const buffer = new ArrayBuffer(8);
+    const view = new DataView(buffer);
+    view.setUint32(0, 0x00000001, false);
+    view.setUint32(4, 0x00000002, false);
+
+    expect(readAdditionalInfoLength(view, 0, true, "Layr"))
+      .toEqual({ length: 0x100000002, bytesRead: 8 });
+    expect(usesEightBytePsbAdditionalInfoLength("Lr16")).toBe(true);
+    expect(usesEightBytePsbAdditionalInfoLength("PxSD")).toBe(true);
+  });
+
+  it("uses 4-byte lengths for PSD regardless of key", () => {
+    const buffer = new ArrayBuffer(8);
+    const view = new DataView(buffer);
+    view.setUint32(0, 1234, false);
+    view.setUint32(4, 5678, false);
+
+    expect(readAdditionalInfoLength(view, 0, false, "Layr"))
+      .toEqual({ length: 1234, bytesRead: 4 });
+  });
+});
diff --git a/src/lib/psbAdditionalInfoLength.ts b/src/lib/psbAdditionalInfoLength.ts
new file mode 100644
index 0000000..2737a0e
--- /dev/null
+++ b/src/lib/psbAdditionalInfoLength.ts
@@ -0,0 +1,34 @@
+const PSB_EIGHT_BYTE_LENGTH_KEYS = new Set([
+  "LMsk",
+  "Lr16",
+  "Lr32",
+  "Layr",
+  "Mt16",
+  "Mt32",
+  "Mtrn",
+  "Alph",
+  "FMsk",
+  "lnk2",
+  "FEid",
+  "FXid",
+  "PxSD",
+]);
+
+export function usesEightBytePsbAdditionalInfoLength(key: string): boolean {
+  return PSB_EIGHT_BYTE_LENGTH_KEYS.has(key);
+}
+
+export function readAdditionalInfoLength(
+  view: DataView,
+  offset: number,
+  isPsb: boolean,
+  key: string,
+): { length: number; bytesRead: 4 | 8 } {
+  if (isPsb && usesEightBytePsbAdditionalInfoLength(key)) {
+    const high = view.getUint32(offset, false);
+    const low = view.getUint32(offset + 4, false);
+    return { length: high * 0x100000000 + low, bytesRead: 8 };
+  }
+
+  return { length: view.getUint32(offset, false), bytesRead: 4 };
+}
diff --git a/src/lib/psdSmartObjectDetection.ts b/src/lib/psdSmartObjectDetection.ts
index a87daa1..cf02bd7 100644
--- a/src/lib/psdSmartObjectDetection.ts
+++ b/src/lib/psdSmartObjectDetection.ts
@@ -1,3 +1,5 @@
+import { readAdditionalInfoLength } from "./psbAdditionalInfoLength";
+
 export interface SmartObjectBounds {
   left: number;
   top: number;
@@ -216,8 +218,10 @@ export async function detectSmartObjectNameFromPsd(file: File): Promise<SmartObj
         const key = readAscii(view, offset + 4, 4);
         offset += 8;
         if (sig !== "8BIM" && sig !== "8B64") break;
-        const dataLength = isPsb ? readUint64AsNumber(view, offset) : view.getUint32(offset, false);
-        offset += isPsb ? 8 : 4;
+
+        const lengthInfo = readAdditionalInfoLength(view, offset, isPsb, key);
+        const dataLength = lengthInfo.length;
+        offset += lengthInfo.bytesRead;
         const dataStart = offset;
         const dataEnd = Math.min(extraEnd, dataStart + dataLength);