Diamond layers were deposited by means of hot-filament chemical vapor deposition from CO/CH 4 /H 2 gas mixtures. With additions of 1 and 5% CO to the CH 4 /H 2 mixtures it was possible to obtain faceted diamond layers. CH 4 concentrations above 1 5% led to ballas diamond formation. Additions between 10 and 15% CO led to faceted diamond layers with methane concentrations up to 2 0%; these experiments resulted in the highest growth rate of 1 68 μm/h. Ballas deposition was observed at 2 5% CH 4 and above. In conclusion, the addition of CO to the CH 4 2 gas mixture can increase the growth rates of faceted diamond. This can be explained by the reacativity of oxygen, which obviously reacts with hydrogen under the given diamond deposition conditions.