A mixture of Ce+3 salt and an aminomethylene phosphonic acid, such as amino tri(methylene phosphonic acid) (ATMP), diethylene triamine penta(methylene phosphonic acid), N,N-di(methylene phosphonic acid) ethanol amine, N,N-di(methylene phosphonic acid)-N-methylamine, N-oxo-N,N,N-tri(methylene phosphonic acid), or 1-hydroxy-ethylidene-1,1-diphosphonic acid, was used for the photopolymerization of acrylonitrile, vinyl acetate, acrylic acid, and styrene in water. Molecular weights of the polymers decreased with increasing concentration of both Ce+3 salt and ATMP. The effect of oxygen, light, pH, and the addition order on polymerization were also studied.