Diamond films grown by rf plasma‐enhanced chemical vapor deposition in dilute CO, CF4, and CH4 (diluent H2) mixtures have been examined by cathodoluminescence (CL) in a transmission electron microscope to assess the incorporation of optically active impurities and defects. The details of the CL spectra are found to be dependent on the different gas mixtures and are correlated with the different film microstructures. Dislocation‐related band A CL due to closely spaced donor‐acceptor (D‐A) pairs was observed from both the CO and CH4‐grown films, but was absent in the CF4‐grown material. Band A CL due to widely separated (D‐A) pairs was seen in all samples but was especially dominant in the CF4‐grown film. Emission due to a di‐Si interstitial impurity was observed in CO‐ and CF4‐grown films but was absent in the CH4‐grown material.