A process is described for carrying out aqueous and non-aqueous polymerization of polymerizable monomers, particularly vinyl monomers, and, preferably, vinylpyrrolidone, in the presence of a mixture of a stable, high purity, free-flowing, fine white powder of a substantially anhydrous 1:1 molar ratio complex of polyvinylpyrrolidone-hydrogen peroxide (PVP--H.sub.2 O.sub.2), respectively, and a transition metallic ion, as a free radical initiator, is described herein. Preferred polymerizations are the homopolymerization of vinylpyrrolidone and the copolymerization of maleic anhydride and a C.sub.1 -C.sub.5 alkyl vinyl ether.