The sensitized polymerizations of styrene, vinyl acetate (VA), acrylonitrile (AN), methyl methacrylate (MMA) and ethyl acrylate (EA) in the presence of benzoquinone (BQ), 2,5-dichlorobenzoquinone(Dich.BQ), chloranil and duroquinone (DQ) have been studied by isolation and investigation of the polymers formed during the induction periods. Quinonoid polymeric derivatives of BQ and Dich.BQ, were obtained for all monomers. In the case of chloranil, quinonoid products were formed only with MMA and EA. Most of the DQ was recovered intact in all systems; only in the case of styrene could quinonoid polymers be detected. The results indicate that substitution of the polymer chains into the quinone nucleus is the predominant mechanism for retardation and inhibition. The feasibility of substitution is determined by the electron-donating power of the polymeric radical, the redox potential of the quinone and the presence of hydrogen atoms in its nucleus. The tendency of the acrylate radicals for disproportionation accounts for the formation of quinonoid products with chloranil. The suggested mechanisms account for the variety of observations on inhibition reactions.