Diethyl-2,3-dicyano-2,3-di-(X-substitute phenyl)succinates(X = p-OCH3, p-CH3, p-Cl, H, p-NO,) can initiate the free-radical polymerization of styrene. The decomposition rate constant and activation energy were measured by means of a dilatometer, and the results showed that they were strongly dependent on the ratio of meso- and dl-isomers in the polysubstituted dibenzyl compounds and the properties of the ring substituents. On the other hand, it was found that the polystyrenes, which were obtained by initiation with hydrogen, had a much larger average molecular weight than that with p-OCH3 and p-CH3. The experimental phenomena were correlated with the structure of the radical resulting from hydrogen.