The kinetics aspects of the photoinitiated polymerization of tetrahydrofuran with triphenylsulfonium hexafluoroarsenate and sensitized by phenothiazine was studied under visible-light irradiation. A photosensitizer mechanism was proposed involving electron transfer from phenothiazine to the triphenylsulfonium salt, forming a cation radical, the true precursor of the polymerization. An initial complex in the ground state between the phenothiazine and the triphenylsulfonium was formed prior to the excitation, and this complex actually activated the photopolymerization rate. The polymerization conversion presented an S-shaped curve, suggesting an induction period caused by the existence of two pathways responsible for the beginning of the polymerization. (C) 2000 John Wiley & Sons, Inc.