Free radicals produced in the photoinduced electron transfer from triethanolamine to excited riboflavin lead to the polymerization of 2-hydroxyethyl methacrylate in aqueous medium. Polymerization rates increase with the amine concentration, reaching a maximun value at 0.01 M amine. Further amine addition produces a decrease of the polymerization. Time-resolved photolysis studies of riboflavin were carried out under the polymerization conditions, monomer/water pH 9. These results indicate that the polymerization proceeds by the radicals formed in the interaction of the dye triplet with the amine. Meanwhile, the quenching of the excited singlet inhibits the polymerization. These results are discussed in terms of the dependence of the photoinitiation mechanism with the solvent employed in the polymerization.