The cationic bulk polymerization of oxiranes such as cyclohexene oxide and 4-vinyl cyclohexene dioxide can be induced by light in the visible wavelength range with the aid of an initiator system consisting of an onium salt and a coumarin (appropriate for applications of lasers emitting light between 400 and 500 nm). This was demonstrated by irradiating neat monomers containing either coumarin 153 (C-153) or coumarin 7 (C-7) and isoquinolinium hexafluorophosphate, diphenyliodonium hexafluorophosphate or triphenylsulfonium hexafluorophosphate with a continuous laser beam (lambda(inc) = 488 nm). The monomer conversion was limited to ca. 60% due to consumption of the coumarin and insolubility of the onium salt in the polymerizing medium at lower monomer content levels. Initiator fragments were not incorporated into the polymer. In the presence of onium salts, the photolysis of coumarins in the absence of O-2 results in proton formation : phi(H+) = 1.3 x 10(-2) (C-7) and phi(H+) = 2.3 x 10(-2) (C-153) and it is concluded that protons play a major role in the initiation of the polymerization.