Information on the outgassing properties of a number of plastic materials has been obtained by the use of a bakeable mass spectrometer. This was fitted with vacuum locks to avoid affecting the residual gas compositions by introduction of the sample. Each material was examined after, (1) it had been pumped for an acceptable pumpdown time, (2) it had been baked in vacuo at its maximum permissible temperature, (3) re-exposure to the laboratory atmosphere followed by the same period of pumping as originally adopted. The majority of the materials examined showed water to be the main component of outgassing. In general, bakeout in vacuo produced a permanent reduction in hydrocarbon evolution, but after re-exposure to laboratory air the rate of water vapor evolution was comparable with that originally observed.