- The deposition temperature dependence of the cathodoluminescence (CL) of diamond thin films grown by microwave plasma‐assisted chemical vapor deposition has been investigated. Depositions were made in the temperature range of 400 to 750 °C at a pressure of 10 Torr, with a gas mixture of 5% CH4 and 5% O2 in hydrogen. The intensity of the luminescent peak at 430 nm was used as a measure of diamond quality for the film. This peak was found to be a maximum above 600 °C. Examination of the intensities of CL emissions associated with nitrogen and silicon impurities at 530, 560, and 740 nm indicate incorporation of these impurities is more efficient at temperatures above 600 °C. Film quality was thus found to be an optimization of competing mechanisms, i.e., improvement of diamond quality as evidenced by the intensity of the 430 nm peak, with the apparent activation of impurities and vacancy defects at elevated temperatures.