A new method for elucidating elementary reaction steps in zeolite catalysis was demonstrated for reactions of isobutene and other monoolefins on HZSM5. Radiolysis was used to "spin label" reaction products, and EPR was used for product analysis. The evolution of products was revealed by quenching the reactions cryogenically in a series of equilibration experiments. Isobutene dimerization and isomerization occur on HZSM5 even at 77 K, and cracking occurs below room temperature. The radiolysis/EPR results allow a consistent interpretation of the literature on EPR signals that arise spontaneously upon interaction of acyclic olefins with air-activated H-Mordenite and HZSM5.