diff --git a/.github/workflows/experimental-workflow.yml b/.github/workflows/experimental-workflow.yml
index 77525c53..9baa3140 100644
--- a/.github/workflows/experimental-workflow.yml
+++ b/.github/workflows/experimental-workflow.yml
@@ -25,7 +25,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        operating-system: [ubuntu-latest, windows-latest, macos-latest]
+        operating-system: [ubuntu-20.04, ubuntu-latest, windows-latest, macos-latest]
         php-versions: ['8.0']
     env:
       extensions: xml, opcache, pcov
diff --git a/.github/workflows/workflow.yml b/.github/workflows/workflow.yml
index 73b7f90a..6bafe794 100644
--- a/.github/workflows/workflow.yml
+++ b/.github/workflows/workflow.yml
@@ -23,7 +23,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        operating-system: [ubuntu-latest, windows-latest, macos-latest]
+        operating-system: [ubuntu-20.04, ubuntu-latest, windows-latest, macos-latest]
         php-versions: ['5.3', '5.4', '5.5', '5.6', '7.0', '7.1', '7.2', '7.3', '7.4']
     env:
       extensions: xml, opcache, xdebug, pcov