The thermal decomposition of benzoyl peroxide of 98 per cent purity has been studied, at atmospheric pressure, at a series of temperatures between 70 and 95 ºC. A liquid phase appears during the decomposition. The point at which this happens, and the rate of decomposition in the early stages, depend on the conservation within the system of slightly volatile products such as benzoic acid and diphenyl which, initially, may be lost by vapour phase diffusion at a
rate comparable with their rate of formation. Results are correlated, over the whole course of the decomposition, by a combination of conventional rate equations and an approximation to the phase equilibrium between undecomposed peroxide and condensed products.