The polymerization of vinyl monomers such as styrene, methyl methacrylate, acrylonitrile, vinyl acetate, and vinyl chloride was carried out in the presence of water with nickel peroxide as a solid radical initiator. It was found that the rates of polymerizations of the monomers markedly increased with the presence of water, except in the case of styrene, and the molecular weights of the polymers obtained in water were higher than those obtained in toluene. The accelerating effect of water was closely related to the solubility of the monomer used in water. The stereoregularities of the polymers resulting from methyl methacrylate, however, were observed to be rather random. The effects of some organic and inorganic reducing agents on the rate of polymerization of methyl methacrylate or styrene initiated by nickel peroxide, and on the molecular weight and the stereoregularity of the resulting polymers were also investigated.