Ferrocene (Fc)-sensitized photopolymerizations of vinyl monomers such as methyl methacrylate (MMA), acrylonitrile (AN), styrene (St), and isobutyl vinyl ether (IBVE) have been investigated by means of kinetic and spectroscopic analyses. The sensitizing effect of Fc has been found to be specific for AN; Fc acts as a sensitizer for AN but not for the other monomers. In the presence of carbon tetrachloride (CCl4), Fc is an effective sensitizer for MMA, AN, and IBVE, but it acts as an inhibitor for St. The polymerizations of AN by Fc and of MMA by Fc/CCl4proceed by a radical mechanism, whereas the polymerization of IBVE by Fc/CCl4proceeds in a catonic fashion. Kinetic studies gave the following relationships:Rp=k1[Fc]0.38[AN]1.7for the polymerization of AN by Fc in dimethylformamide;Rp=K2[Fc]0.48[CCl4]0.42[MMA]1.5for the polymerization of MMA by Fc/CCl4in benzene. Spectroscopic analysis indicated that the charge-transfer complexes are formed between Fc and a monomer or CCl4. The initiating species of the polymerizations are produced by the photochemical dissociation of the complexes.