The polymerization of styrenesulfonate photoinitiated with visable light in the presence of the dye safranine (lambda (max) = 532 nm) has been studied at various monomer concentrations in aqueous solution (>0.1 M). The elemental photochemical and ground state steps involved in the initiation were studied individually in order to obtain a general mechanism for the photoinitiation. Two important facts were found: first, the photopolymerization proceeds in the presence of air, whereas co-inhibitors like amines, inhibit the reaction: second, several evidences obtained from ground state excited-state interactions, as well as from the polymerization products suggest the aggregation of the monomer in clusters containing a dye molecule. Similar evidences were found when using the nonpolymerizable anion toluenesulfonate.