Abstract: Granules of azobisamidine salt can be produced by granulating a powder of azobisamidine salt using a limited amount of water or a mixture of water and a water-soluble organic solvent such as methanol are convenient in handling without deteriorating the quality and can be used as a water-soluble polymerization initiator.

Abstract: An azo polymerization initiator composition obtained by kneading an azo compound, a dispersing agent and a surface active agent at a temperature of 20.degree. C. or lower, while adjusting the water content of the composition to 0 to 50% by weight, has good storage stability for a long period of time and, at any time desired for polymerization, can give a uniform aqueous suspension of the azo compound for polymerization of unsaturated vinyl monomers with high polymerization initiator activity.

Abstract: A stable aqueous suspension containing an organic azo compound as polymerization initiator in fine particles, containing substantially no organic solvent and containing no foam which damages dispersing stability of the aqueous suspension and retards polymerization rate can be formed by dissolving an organic azo compound in an organic solvent, emulsifying the resulting solution by adding water and a dispersing agent A and a dispersing agent B thereto with stirring, and removing the organic solvent from the emulsified liquid preferably under reduced pressure.