diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 78e0047d3f..8b822b5c69 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -20,7 +20,7 @@ jobs:
     strategy:
       fail-fast: false # do not cancel builds for other OSes if one fails
       matrix:
-        os: [ "ubuntu-latest", "macOS-latest", "windows-latest" ]
+        os: [ "ubuntu-18.04", "macOS-10.15", "windows-2016" ]
 
     runs-on: "${{ matrix.os }}"
 
diff --git a/CHANGELOG.md b/CHANGELOG.md
index fecde031cb..8414db8a2f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -14,6 +14,8 @@ Bugfixes:
 
 * Allow fixity, kind, role declarations in REPL (#4046, @rhendric)
 
+* Pin OS versions used in CI (#4107, @f-f)
+
 Internal:
 
 * Fix for Haddock (#4072 by @ncaq and @JordanMartinez)
diff --git a/CONTRIBUTORS.md b/CONTRIBUTORS.md
index 2eaf3a639d..1dd3ff21f9 100644
--- a/CONTRIBUTORS.md
+++ b/CONTRIBUTORS.md
@@ -52,6 +52,7 @@ If you would prefer to use different terms, please use the section below instead
 | [@felixSchl](https://github.com/felixSchl) | Felix Schlitter | [MIT license](http://opensource.org/licenses/MIT) |
 | [@FrigoEU](https://github.com/FrigoEU) | Simon Van Casteren | [MIT license](http://opensource.org/licenses/MIT) |
 | [@fsoikin](https://github.com/fsoikin) | Fyodor Soikin | [MIT license](http://opensource.org/licenses/MIT) |
+| [@f-f](https://github.com/f-f) | Fabrizio Ferrai | [MIT license](http://opensource.org/licenses/MIT) |
 | [@garyb](https://github.com/garyb) | Gary Burgess | [MIT license](http://opensource.org/licenses/MIT) |
 | [@hdgarrood](https://github.com/hdgarrood) | Harry Garrood | [MIT license](http://opensource.org/licenses/MIT) |
 | [@houli](https://github.com/houli) | Eoin Houlihan | [MIT license](http://opensource.org/licenses/MIT) |