The kinetics of the decomposition of hydrogen peroxide in the vapour state have been studied by a manometric method, with pure hydrogen peroxide at a concentration of about 99.5%. The temperature coefficient of the reaction has been measured for the first time. The pressures ranged from 1 to 2 cm. of mercury and the temperatures from 70° to 200 °C. Pyrex reaction vessels of various sizes and shapes, and also a fused Pyrex and a soda-glass vessel, were used. The reaction was purely heterogeneous, of the first order up to 140 °C. but more complicated at higher temperatures. Identical vessels yielded consistent results. The rates were not affected by air, carbon dioxide, or water vapour, but they varied greatly with the size and shape of the vessel. The reaction was very slow on fused Pyrex and very rapid on soda-glass. In one vessel, the temperature coefficient became negligible above 120 °C. No explosion was detected up to 335 °C. at a pressure of 18 cm. of mercury. The apparent activation energies in various vessels ranged from 13.5 to 18.5 kcal. per mole. A tentative reaction mechanism is suggested.