With the aid of a mass spectrometer a study has been made of the gaseous contaminants present when oxygen is exposed to different incandescent filaments. In agreement with others CO and CO2were the most abundant contaminants found. The amounts of CO and CO2found, at a given filament temperature and oxygen pressure, were greater for Mo 0.031% C, W 0.012% C, and Re 0.010% C than for Ta 0.001% C. Also at the same oxygen pressure and filament temperature the quantity of CO found was much greater for a carbonized W filament than for any of the other filaments investigated. These results are consistent with the hypothesis that CO and CO2are produced by the interaction of oxygen with the carbon present in the hot filaments. No evidence was found to indicate that the glass walls contribute to the formation of CO and CO2.