The Bronsted acid-catalyzed transformations of cyclic olefins in zeolites HZSM5 and H-mordenite were elucidated by the radiolysis/EPR method. Products of catalytic reactions were spin-labeled by radiolytic means and identified by using EPR spectroscopy. Comparison of reactions on HZSM5 and H-mordenite reveals strong shape selectivity on reactions in these catalysts. The radiolysis/EPR method is a new in situ spectroscopy with excellent sensitivity and structural specificity for the study of reaction mechanisms in zeolite catalysts.