The photopolymerization of acrylamide (AA) initiated by the metallic complex tris(2,2'-bipyridine)ruthenium(II) [Ru(bPY)(3)(+2)] in the presence of aliphatic and aromatic amines as co-initiators was investigated in aqueous solution. Aromatic amines, which are good quenchers of the emission of the metal-to-ligand-charge-transfer excited state of the complex, are more effective co-initiators than those that do not quench the luminescence of Ru(bpy)(3)(+2), such as aliphatic amines and aniline. Laser-flash photolysis experiments show the presence of the reduced form of the complex, Ru(bpy)(3)(+1), for all the amines investigated. For aliphatic amines, the yield of Ru(bpy)(3)(+1) increases with temperature, and on the basis of these experiments, a metal-centered excited state is proposed as the reactive intermediate in the reaction with these amines. The decay of the transient Ru(bpy)(3)(+1) is faster in the presence of AA. This may be understood by an electron-transfer process from Ru(bpy)(3)(+1) to AA, regenerating Ru(bpy)(3)(+2) and producing the radical anion of AA. It is proposed that this radical anion protonates in a fast process to give the neutral AA radical, initiating in this way the polymerization chain.