diff --git a/.github/workflows/CI.yaml b/.github/workflows/CI.yaml
index bce1da0..cdf09d8 100644
--- a/.github/workflows/CI.yaml
+++ b/.github/workflows/CI.yaml
@@ -25,15 +25,25 @@ jobs:
       matrix:
         os: [macOS-latest, ubuntu-latest, windows-latest]
         python-version: ["2.7", "3.5", "3.6", "3.7", "3.8", "3.9", "3.10"]
+        # Only test lowest and highest version on the expensive/slow
+        # macOS and windows runners (UPDATE when supported versions change):
         exclude:
-        - os: macOS-latest
-          python-version: 3.5	
         - os: macOS-latest
           python-version: 3.6
         - os: macOS-latest
           python-version: 3.7
         - os: macOS-latest
           python-version: 3.8
+        - os: macOS-latest
+          python-version: 3.9
+        - os: windows-latest
+          python-version: 3.6
+        - os: windows-latest
+          python-version: 3.7
+        - os: windows-latest
+          python-version: 3.8
+        - os: windows-latest
+          python-version: 3.9
 
     steps:
     - uses: actions/checkout@v2
@@ -78,3 +88,31 @@ jobs:
         flags: unittests
         fail_ci_if_error: true
 
+
+  pypi_check:
+    name: Check source package integrity (for PyPi deployment)
+    if: "github.repository == 'alchemistry/alchemtest'"
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+
+    - name: Setup Python
+      uses: actions/setup-python@v2
+      with:
+        python-version: 3.9
+
+    - name: Install tools
+      run: |
+        python -m pip install setuptools twine
+        
+    - name: Build alchemtest source dist
+      run: |
+        python setup.py sdist
+        
+    - name: Check package build sdist
+      run: |
+        DISTRIBUTION=$(ls -t1 dist/alchemtest-*.tar.gz | head -n 1)
+        test -n "${DISTRIBUTION}" || { echo "no distribution dist/alchemtest-*.tar.gz found"; exit 1; }
+        echo "twine check $DISTRIBUTION"
+        twine check $DISTRIBUTION