The cationic polymerization of cyclohexene oxide (CHO) was achieved by UV irradiation (lambda = 350-420 nm) of methylene chloride solutions containing benzoyltrimethylgermane (BTG) and onium salts, such as diphenyliodonium hexafluorophosphate (Ph2I+PF6-) or N-ethoxy-2-methylpyridinium hexafluorophosphate (EMP+PF6-). A feasible initiation mechanism involves the photogeneration of germyl radicals and benzoyl radicals in the first step. Subsequent oxidation of germyl radicals by onium salts yields germanium ions capable of initiating the polymerization of CHO. In agreement with the proposed mechanism, the polymerization was completely inhibited by a radical scavanger such as 2,2,6,6-tetramethylpiperidinyl-1-oxy, and polymerization efficiency was directly related to the reduction potential of the onium salts, i.e. Ph(2)l+PF6- (E-red(1/2) = -0.2 V) was found to be more efficient than EMP+PF6- (E-red(1/2) = -0.7 V). The results were compared to the other photoinitiators generally used in free radical promoted cationic polymerizations. In addition to CHO, vinyl monomers such butyl vinyl ether and N-vinyl carbazole, and a bisepoxide Such as 3,4-epoxycyclohexyl-3 ',4 '-epoxycyclohexene carboxylate, were polymerized in the presence of BTG and iodonium salt with high efficiency.