diff --git a/metropolis/pkg/efivarfs/format.go b/metropolis/pkg/efivarfs/format.go
new file mode 100644
index 0000000..a71dbf9
--- /dev/null
+++ b/metropolis/pkg/efivarfs/format.go
@@ -0,0 +1,33 @@
+// Copyright 2020 The Monogon Project Authors.
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package efivarfs
+
+import (
+	"github.com/google/uuid"
+)
+
+// MarshalEFIGUID returns src represented in the EFI GUID format, as defined in
+// UEFI specification, version 2.9, appendix A:
+// https://uefi.org/sites/default/files/resources/UEFI_Spec_2_9_2021_03_18.pdf
+func MarshalEFIGUID(src uuid.UUID) []byte {
+	efi := make([]byte, 16)
+	transform := []int{3, 2, 1, 0, 5, 4, 7, 6, 8, 9, 10, 11, 12, 13, 14, 15}
+	for dest, from := range transform {
+		efi[dest] = src[from]
+	}
+	return efi
+}