diff --git a/src/compound/union_test.ts b/src/compound/union_test.ts
new file mode 100644
index 0000000..e2a72fc
--- /dev/null
+++ b/src/compound/union_test.ts
@@ -0,0 +1,57 @@
+import { u32le, u8 } from "../mod.ts";
+import { assertEquals, assertThrows } from "../../test_deps.ts";
+import { Union } from "./union.ts";
+
+Deno.test({
+  name: "Union",
+  fn: async (t) => {
+    const ab = new ArrayBuffer(8);
+    const dt = new DataView(ab);
+    const type = new Union({
+      0: u32le,
+      1: u8,
+      2: u8,
+    }, (a) => a === 32 ? 0 : 1);
+
+    await t.step("Read", () => {
+      dt.setUint8(0, 1);
+      dt.setUint8(1, 11);
+      dt.setUint8(2, 22);
+      dt.setUint8(4, 33);
+      const result = type.read(dt);
+      assertEquals(result, 33);
+    });
+
+    await t.step("Read Packed", () => {
+      dt.setUint8(0, 1);
+      dt.setUint8(1, 11);
+      dt.setUint8(2, 22);
+      dt.setUint8(4, 33);
+      const result = type.readPacked(dt);
+      assertEquals(result, 11);
+    });
+
+    dt.setBigUint64(0, 0n);
+
+    await t.step("Write", () => {
+      type.write(32, dt);
+      assertEquals(new Uint32Array(ab), Uint32Array.of(0, 32));
+    });
+
+    dt.setBigUint64(0, 0n);
+
+    await t.step("Write Packed", () => {
+      type.write(32, dt);
+      assertEquals(
+        new Uint8Array(ab).subarray(0, 5),
+        Uint8Array.of(0, 0, 0, 0, 32),
+      );
+    });
+
+    await t.step("OOB Read", () => {
+      assertThrows(() => {
+        type.read(dt, { byteOffset: 9 });
+      }, RangeError);
+    });
+  },
+});