Variable-temperature EPR was used to investigate reactions of olefin radical cations generated radiolytically in nonacidic and acidic ZSM-5 zeolites. The olefin radical cations undergo isomerization reactions even at 4 K. Radical cation reactions are presumably driven by the exothermicity of charge transfer, which is not efficiently quenched by the vibrational modes of the zeolite lattice. The observation of H-addition type radicals indicates Bronsted acid-catalyzed rearrangements prior to irradiation on the more acidic zeolites.