Polymerization rates of 2-hydroxyethyl methacrylate (HEMA) photoinitiated by riboflavin were investigated using as co-initiator several amines of different structure. The photoinitiation efficiency is highly dependent on the structure of the amine. The photochemical behavior of these systems was investigated under the polymerization conditions. The results are explained in terms of the dependence of the amino radicals production and the reactivity of these radicals toward monomer double bonds with the structural features of the amines.