Analysis of both major and minor products of the thermal reaction of mixtures of propylene and acetaldehyde over the temperature range 750-800 "K and a t pressures below atmospheric show that both reactants initiate radical chains and that both addition and abstraction reactions of radicals with propylene are important. The relation to general mechanisms of the inhibiting effect of propylene in pyrolysis reactions is discussed. Introduction Propylene has long been known to inhibit the decomposition of many organic compounds (1-4). The inhibition has been explained by both addition and abstraction reactions of radicals with propylene. Both reactions reduce the apparent rate of reaction, the former by the formation of higher molecular weight products which lowers the rate of pressure increase, and the latter by the substitution of the relatively non-reactive allyl radical for the more reactive chain-carrying radical. A general mechanism has been formulated for such systems by Niclause et al. (5), who developed equations describing the rate of decomposition of an organic compound as a function of the concentration of additive for various types of chain mechanisms. With propylene they assumed that the predominant effect is the substitution of the allyl radical for the normal propagating radical, and for simplicity neglected addition reactions, as had been done by other authors (6, 7), although it was acknowledged that they could have some importance. Addition reactions were considered by Quinn (8) and by Marta et al. (9) in their studies of the effect of propylene on the pyrolysis of butane and of propionaldehyde, respectively, but their importance could not be assessed with any certainty because of incomplete product analysis. The relative importance of addition and abstraction reactions is a basic question in the mechanism of inhibition by propylene and in the related effect of self-inhibition of the pyrolysis of paraffins by the prod-