Analysis of the products of the pyrolysis of propylene in the initial stages shows that products of molecular weight up to C9 are primary products of major significance below about 850 °K and above about 200 Torr. As these products accumulate in the system, their dissociation causes secondary initiation and a rapid increase in pressure. Measurements of the initial rate of decrease in pressure and of the final maximum rate of increase in pressure of the reaction are consistent with this interpretation of the mechanism.