Photopolymerization of methyl methacrylate in the presence of mercaptobenzthiazole shows some deviation from the simple reaction scheme, particularly at higher initiator concentrations and with solvents like benzene. The deviation has been explained by the participation of mercaptobenzthiazole in the thiol-thione tautomeric form. Kinetic observations are consistent with a mechanism involving an equilibrium complex between thiol and thione structures. Spectrophotometric studies on initiator decomposition and direct measurement of rates of initiation show relatively high initiator efficiency. © 1989.