An equation is derived which gives the accommodation coefficient $\alpha$ of a gas striking a surface as the ratio of the observed heat loss from the surface to the theoretical heat loss that would be observed if all the gas molecules came to thermal equilibrium with the surface. The experiments show that at temperatures above 600°K the values of $\alpha$ for hydrogen (0.20 mm pressure) are greatly reduced by the presence of oxygen on the surface of the tungsten. Oxygen is inevitably produced in a tube when a tungsten filament is burned at $T>\mathrm{}1500\mathrm{}°$K in hydrogen, as the atomic H thus formed dislodges oxygen from the walls even when the walls are cooled in liquid air. Hydrogen is adsorbed on tungsten at $T<\mathrm{}1200\mathrm{}°$K in two different forms, both of which reduce $\alpha$ from its value for bare tungsten. A film of the first type, which is adsorbed at $T<\mathrm{}600\mathrm{}°$K, changes over into the second type slowly at 600°K and rapidly at 1100°K. The numerical values of $\alpha$ range from 0.537 for bare tungsten to 0.143 for tungsten with an adsorbed hydrogen film of the second type, and 0.094 for tungsten with an adsorbed film of oxygen. At $T<\mathrm{}200\mathrm{}°$K an oxygen film forms which increases $\alpha$ to 0.422 at 150°K, provided that a small concentration of oxygen is continually present in the gas phase.

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