diff --git a/platform/posix/coverage.py b/platform/posix/coverage.py
index 3075b2c0f..e4be1c517 100755
--- a/platform/posix/coverage.py
+++ b/platform/posix/coverage.py
@@ -31,11 +31,12 @@ def clean():
     if not sourcedirs:
         return
 
-    command = ['lcov', '--zerocounters']
+    dirflags = []
     for d in sourcedirs:
-        command.append('-d')
-        command.append(d)
-    subprocess.call(command)
+        dirflags.append('-d')
+        dirflags.append(d)
+    subprocess.call(['lcov', '--zerocounters'] + dirflags)
+    subprocess.call(['lcov', '-c', '-i', '-b', '.'] + dirflags + ['-o', 'coverage-base.info'])
 
 
 def test():
@@ -63,13 +64,21 @@ def collect():
         command.append(d)
     subprocess.call(command)
 
+    # Add the baseline
+    subprocess.call(['lcov',
+                    '-a', 'coverage-raw.info',
+                    '-a', 'coverage-base.info',
+                    '-o', 'coverage-tmp.info'])
+
     # Filter to remove non-Cantera code
     subprocess.call(['lcov',
                      '-o', 'coverage.info',
-                     '-e', 'coverage-raw.info',
+                     '-e', 'coverage-tmp.info',
                      os.getcwd() + '/include/*',
                      os.getcwd() + '/src/*'])
     os.remove('coverage-raw.info')
+    os.remove('coverage-base.info')
+    os.remove('coverage-tmp.info')
 
 def genhtml():
     """