diff --git a/.github/workflows/chm.yml b/.github/workflows/chm.yml
new file mode 100644
index 000000000..2255c4d8f
--- /dev/null
+++ b/.github/workflows/chm.yml
@@ -0,0 +1,85 @@
+name: chm
+
+on:
+  push:
+    branches: 
+      - master
+      - develop
+    paths-ignore:
+      - 'doc/**'
+      - 'Version3/**'
+      - 'README.md'
+  pull_request:
+    branches: 
+      - master
+      - develop
+    paths-ignore:
+      - 'doc/**'
+      - 'Version3/**'
+      - 'README.md'
+  schedule:
+    - cron: '1 2 2/7 * *'
+  workflow_dispatch:
+
+jobs:
+  build-chm:
+    runs-on: windows-latest
+    steps:
+
+    - name: '🧰 Checkout'
+      uses: actions/checkout@v4
+
+    - name: 'fixup_html'
+      shell: bash
+      working-directory: ${{ github.workspace }}/HTMLversion/HTML
+      run: ./fixup_html.bash
+
+    - name: cache htmlhelp
+      id: cache-htmlhelp
+      uses: actions/cache@v4
+      with:
+        path: "C:/Program Files (x86)/HTML Help Workshop"
+        key: ${{ runner.os }}-htmlhelp
+
+    - name: install htmlhelp
+      if: steps.cache-htmlhelp.outputs.cache-hit != 'true'
+      timeout-minutes: 5
+      run: |
+        curl -L -O --max-time 120 http://web.archive.org/web/20160201063255/http://download.microsoft.com/download/0/A/9/0A939EF6-E31C-430F-A3DF-DFAE7960D564/htmlhelp.exe
+        cmd /c start /wait .\htmlhelp.exe /q /c /t:$(Get-Location)\tmp
+        $file = "$(Get-Location)\tmp\htmlhelp.inf"
+        (Get-Content $file) | Foreach-Object { $_ `
+          -replace '^BeginPrompt',                                               ';;BeginPrompt' `
+          -replace '^EndPrompt',                                                 ';;EndPrompt' `
+          -replace '^49000=CustomLDID49000, 1',                                  '49000=CustomLDID49000, 5' `
+          -replace '^"hhupd.exe',                                                ';;hhupd.exe' `
+          -replace '^DefaultInstallDir="C:\\Program Files\\HTML Help Workshop"', 'DefaultInstallDir="%ProgramFiles%\\HTML Help Workshop"'
+        } | Set-Content $file
+        cmd /c start /wait .\tmp\setup.exe
+        Remove-Item -Recurse -Force .\tmp
+        Remove-Item -Recurse -Force .\htmlhelp.exe
+
+    - name: add htmlhelp to path
+      run: Write-Output 'C:\Program Files (x86)\HTML Help Workshop' | Out-File -FilePath $env:GITHUB_PATH -Encoding utf8 -Append
+
+    - name: compile chm
+      shell: bash
+      working-directory: ${{ github.workspace }}/HTMLversion
+      run: |
+        # hhc returns 1 on success
+        set +e
+        hhc phreeqc3.hhp | tee hhc.out
+        status=${PIPESTATUS[0]}
+        set -e
+        if [ "$status" -eq 1 ]; then
+          echo "[OK]"
+          exit 0
+        else
+          echo "[FAILED]"
+          exit 1
+        fi
+
+    - uses: actions/upload-artifact@v4
+      with:
+        name: chm
+        path: ${{github.workspace}}/HTMLversion/phreeqc3.chm