In order to study the correlation between active species formed from styrene by radiation and its radiation-induced polymerization, irradiated organic glasses containing styrene were examined by the electron spin resonance (ESR) method at 77°K. In 2-methyltetrahydrofuran glass, anion radicals of styrene were formed by the transfer of electrons ejected by radiation from the glass matrix molecules to styrene. In n-butylchloride glass, cation radicals of styrene were formed by the transfer of positive charges. The ESR spectrum of the anion radicals is a broad singlet with a width between the maximum slope points of 15 Gs and is not bleached readily by visible light, whereas that of the cation radicals has a width of 23 Gs and is not bleached. The radiation-induced polymerization proceeded in the glass where the cation radicals were present. Otherwise, no polymerization was found. This suggests that the radiation-induced polymerization of styrene, especially in the glassy state, proceeds by a cationic mechanism.