diff --git a/.github/workflows/opam-publish.yml b/.github/workflows/opam-publish.yml
index e85eff64..35c31a8d 100644
--- a/.github/workflows/opam-publish.yml
+++ b/.github/workflows/opam-publish.yml
@@ -11,7 +11,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-latest]
+        os: [ubuntu-18.04]
         ocaml-version: [4.10.0]
 
     steps:
diff --git a/.github/workflows/opam.yml b/.github/workflows/opam.yml
index 6eb34c8f..c137fd95 100644
--- a/.github/workflows/opam.yml
+++ b/.github/workflows/opam.yml
@@ -11,7 +11,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-latest, macos-latest, windows-latest]
+        os: [ubuntu-18.04, macos-latest, windows-latest]
         ocaml-version: [4.11.1]
 
     steps: