Pyrenylmethyl group-containing phosphonium salts (1 and 2) were prepared and their ability as photolatent cationic initiator was examined in the polymerization of epoxides and vinyl monomers. Photopolymerization of reactive epoxides such as cyclohexene oxide (CHO) and styrene oxide (SO) proceeded very well, but some clear termination was observed in the polymerization of less reactive epoxides. In the photopolymerization of vinyl monomers such as vinyl ethers and styrenes, no termination was observed. Re-initiation mechanism, which involved photodecomposition of the phosphine-terminated end of polystyrene to produce a new cationic initiation species, was suggested by the photopolymerization of CHO with benzyl triphenylphosphonium salt as model of the terminated end.