The behaviour of a series of 4-substituted thiophenols as chain transfers in the photopolymerization of the acrylamide and methyl methacrylate was studied. The addition of concentrations lower than 1 mM of thiophenols to the polymerization of acrylamide in aqueous solutions reduces slightly the polymerization rate but decreases markedly the polymer molecular weight. Chain transfer constants were determined by a modified Mayo equation that consider the variation of the polymerization rate with the chain transfer agent. The addition of thiophenols to the polymerization of methyl methacrylate in organic media reduced considerably the polymer molecular weight without variations in the polymerization rate. The chain transfer rate constants for both monomers are well correlated with the electron donor-acceptor ability of the 4-substitutents, increasing with electron donating groups. An amino substituent in the 4-position increases 10 times the chain transfer constant with respect to the unsubstituted compound. No correlations were found with reported data on stability of the thiyl radicals or the addition rate to the monomer. The electron-donating effect of 4-substituents on the chain transfer constant is related to polar structures in the transition state, and suggests that the main controlling chain transfer step by thiophenols is the H-abstraction from the S-H. (c) 2005 Elsevier Ltd. All rights reserved.