Photolysis of the supramolecular complexes (dibenzyl ketones@ZSM-5) produced supramolecular complexes of benzyl radicals@ZSM-5, which were directly detected by CW-EPR spectroscopy, and provided information on the dynamics of the radicals. The lifetimes of the radicals increased as the group X attached to the carbon atom at the radical center increases from X = H (t(1/2) ca. 2 min) to X = (CH(2))(4)CH(3) (t(1/2) > 200 min). In addition, line broadening of the EPR signal was observed as the group X increases. Experiments involving cation-exchanged zeolites (MZSM-5; M = Li, Na, K, Rb, Cs) showed a strong dependence of the radical lifetime on the size of the cation (t(1/2) ca. 10 min for Li and t(1/2) > 200 min for Cs). The results are discussed in terms of supramolecular steric effects on the radical-radical reactions in the zeolite supercages.