diff --git a/mixed-format/fixture/cjs.js b/mixed-format/fixture/cjs.js
new file mode 100644
index 0000000..b7d3583
--- /dev/null
+++ b/mixed-format/fixture/cjs.js
@@ -0,0 +1 @@
+exports.qux = require('./cjs2.js');
diff --git a/mixed-format/fixture/cjs2.js b/mixed-format/fixture/cjs2.js
new file mode 100644
index 0000000..732f419
--- /dev/null
+++ b/mixed-format/fixture/cjs2.js
@@ -0,0 +1 @@
+module.exports = 'zed';
diff --git a/mixed-format/fixture/esm.js b/mixed-format/fixture/esm.js
new file mode 100644
index 0000000..c155820
--- /dev/null
+++ b/mixed-format/fixture/esm.js
@@ -0,0 +1 @@
+export const foo = 'bar';
diff --git a/mixed-format/fixture/package.json b/mixed-format/fixture/package.json
new file mode 100644
index 0000000..a0df0c8
--- /dev/null
+++ b/mixed-format/fixture/package.json
@@ -0,0 +1,3 @@
+{
+	"type": "commonjs"
+}
diff --git a/mixed-format/loader.mjs b/mixed-format/loader.mjs
new file mode 100644
index 0000000..afb903a
--- /dev/null
+++ b/mixed-format/loader.mjs
@@ -0,0 +1,34 @@
+export async function load(url, context, nextLoad) {
+  if (!url.endsWith('.js')) return nextLoad(url);
+
+  const nextResult = await nextLoad(url, { ...context, format: 'module' })
+    .then((result) => {
+      if (containsCJS(result.source)) { throw new Error('CommonJS'); }
+
+      return result;
+    })
+    .catch(async (err) => {
+      if (
+        (err?.message.includes('require') && err.includes('import'))
+        || err?.message.includes('CommonJS')
+      ) {
+        return { format: 'commonjs' };
+      }
+
+      throw err;
+    });
+
+  return nextResult;
+}
+
+function containsCJS(source) {
+  const src = '' + source;
+
+  // A realistic version of this loader would use a parser like Acorn to check for actual `module.exports` syntax
+  if (src.match(/exports[\.( ?=)]/)) { return true };
+
+  if (
+    src.match(/require\(/)
+    && !src.match(/createRequire\(/)
+  ) return true;
+}
diff --git a/mixed-format/test.mjs b/mixed-format/test.mjs
new file mode 100644
index 0000000..616dc86
--- /dev/null
+++ b/mixed-format/test.mjs
@@ -0,0 +1,8 @@
+import assert from 'node:assert';
+
+import { foo } from './fixture/esm.js';
+import { qux } from './fixture/cjs.js';
+
+
+assert.strictEqual(foo, 'bar');
+assert.strictEqual(qux, 'zed');