Chemical reaction products formed in a microwave plasma assisted chemical vapor deposition apparatus for diamond film deposition are detected using mass spectrometry. Carbon source gases CH4, C2H6, C2H4, or C2H2 produce CH4, C2H2, CO, and H2O as major stable reaction products when introduced into a H2/O2 plasma under diamond deposition conditions. The effect of oxygen addition is similar for all carbon source gases with respect to reaction product formation, indicating that a common reaction mechanism is active in all cases. On a qualitative basis, these observations are consistent with a mechanism describing the oxidation of CH4 in flames. No beneficial effects were observed using alternating growth/etch cycles to deposit films. Films grown using CH4 as the carbon source gas consistently produce higher quality diamond films compared to films grown from C2H2.