Polycrystalline diamond films have been deposited on (100) silicon wafers by plasma enhanced chemical vapor deposition (PECVD) using a 1.5 kW microwave source to dissociate dilute gas mixtures of CH4 and O2 in H2. Films as thick as 20μm covering a 2″ diameter area were deposited at 925 °C, 40 Torr total pressure, and 500 sccm total flow. These have been characterized as a function of CH4 [0.2–4%] and Os [0–3%] concentrations by measurements of deposition rate, stress, surface roughness, morphology, and impurity levels (C-H and amorphous-graphitic carbon). Addition of oxygen to the discharge tends to reduce impurity levels in the diamond films; however, this is accompanied by a reduction in deposition rate. When an effective CH4 concentration [= %CH4 - %O2] is used as a metric for O2 -containing feed compositions, deposition rates as well as film properties are found to agree well with those obtained in the absence of O2. Thus, 1% CH4 in hydrogen is nearly equivalent to 4% CH4, 3% O2 in hydrogen as feed compositions for depositing diamond.