Cationic photopolymerization of epoxides and vinyl ethers by the direct and sensitized photolysis of onium tetrakis(pentafluorophenyl)borates as photo-initiators is reviewed. Most of the onium borates are sensitized by the anthracenes (non-substituted-, 9-methyl-, and 9,10-dimethylanthracene). The rates of polymerization increase with decreasing free energy changes between the onium borates and the excited singlet state of the anthracenes. The acid generated by the photodecomposition of the onium borates produces non-acidic products by heating. The photolysis mechanism of the onium borates is also described.