NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. The accommodation coefficients for the gases and metals tested vary rapidly in the range where the gas temperature is within approximately 50°C of the filament temperature. When the temperature excess becomes large, the [...] of argon, nitrogen and helium on platinum is approximately .7, .5 and .2, respectively, and on tungsten approximately .5, .35 and .15, although the actual value depends upon both filament and gas temperature. Variation in pressure up to 150 microns was found to have a negligible effect on [...]. All gases and metals tested gave smooth curves of [...] versus temperature except helium on tungsten at a gas temperature of 80° K., which exhibited a sharp discontinuity at a filament temperature of approximately 225° K. and a pressure of 34 microns.