diff --git a/packages/core-audit/src/pseudonymize.test.ts b/packages/core-audit/src/pseudonymize.test.ts
new file mode 100644
index 0000000..66d737c
--- /dev/null
+++ b/packages/core-audit/src/pseudonymize.test.ts
@@ -0,0 +1,58 @@
+import { describe, it, expect, beforeEach, afterEach } from "vitest";
+import { pseudonymize } from "./pseudonymize";
+
+describe("pseudonymize", () => {
+  const originalSalt = process.env["AUDIT_PSEUDONYM_SALT"];
+
+  beforeEach(() => {
+    process.env["AUDIT_PSEUDONYM_SALT"] = "test-salt-1";
+  });
+
+  afterEach(() => {
+    if (originalSalt === undefined) {
+      delete process.env["AUDIT_PSEUDONYM_SALT"];
+    } else {
+      process.env["AUDIT_PSEUDONYM_SALT"] = originalSalt;
+    }
+  });
+
+  it("returns a string prefixed with 'erased-'", () => {
+    const result = pseudonymize("user_42");
+    expect(result).toMatch(/^erased-/);
+  });
+
+  it("produces exactly 16 hex chars after the prefix", () => {
+    const result = pseudonymize("user_42");
+    const hex = result.slice("erased-".length);
+    expect(hex).toHaveLength(16);
+    expect(hex).toMatch(/^[0-9a-f]+$/);
+  });
+
+  it("is deterministic — same salt + actorId always yields the same token", () => {
+    const a = pseudonymize("user_42");
+    const b = pseudonymize("user_42");
+    expect(a).toBe(b);
+  });
+
+  it("differs when actorId differs (same salt)", () => {
+    const a = pseudonymize("user_42");
+    const b = pseudonymize("user_99");
+    expect(a).not.toBe(b);
+  });
+
+  it("differs when the salt changes", () => {
+    const withSalt1 = pseudonymize("user_42");
+
+    process.env["AUDIT_PSEUDONYM_SALT"] = "test-salt-2";
+    const withSalt2 = pseudonymize("user_42");
+
+    expect(withSalt1).not.toBe(withSalt2);
+  });
+
+  it("uses the fallback salt when env var is absent", () => {
+    delete process.env["AUDIT_PSEUDONYM_SALT"];
+    // Should not throw; just use the fallback.
+    const result = pseudonymize("user_1");
+    expect(result).toMatch(/^erased-[0-9a-f]{16}$/);
+  });
+});
diff --git a/packages/core-audit/src/pseudonymize.ts b/packages/core-audit/src/pseudonymize.ts
new file mode 100644
index 0000000..4439cb1
--- /dev/null
+++ b/packages/core-audit/src/pseudonymize.ts
@@ -0,0 +1,22 @@
+import { createHash } from "node:crypto";
+
+/**
+ * Produces a stable, irreversible token for a GDPR-erased actorId.
+ *
+ * Format: `erased-<first-16-hex-chars-of-sha256(salt:actorId)>`
+ *
+ * The salt is read from `AUDIT_PSEUDONYM_SALT` env at call time so that
+ * production binding can pre-validate the var at boot (see `bindAudit`)
+ * while tests can override it per-test via `process.env`.
+ *
+ * Fallback salt is intentionally weak and labelled so that any token
+ * produced with it is recognisable as a dev/test artefact.
+ */
+export function pseudonymize(actorId: string): string {
+  const salt =
+    process.env["AUDIT_PSEUDONYM_SALT"] ?? "dev-fallback-salt-replace-in-prod";
+  const hash = createHash("sha256")
+    .update(`${salt}:${actorId}`)
+    .digest("hex");
+  return `erased-${hash.slice(0, 16)}`;
+}