Following the adsorption of (Z)- or (E)-piperylene (penta-1,3-diene), penta-1,4-diene or hexa-1,5-diene onto H-mordenite, EPR signals characteristic of 9-octalin radical cations 1/6 were recorded. Comparative studies of these dienes were made in solid freon matrices which indicate that a dominant mechanism involving Bronsted acid catalysis is required to explain the main molecular transformations, prior to radical cation formation.