Abstract: Process for the production of a stable polymer emulsion by polymerizing a radical-polymerizable monomer having an unsaturated bond in the presence of a water-soluble polymer containing monomer units consisting essentially of an ethylenically unsaturated carboxylic acid or salts thereof, and an ethylenically unsaturated sulfonic acid or a salt thereof and an anionic surface active agent. This polymerization takes place in an aqueous medium at a pH not higher than 4 in the presence of a water-soluble catalyst. The polymer emulsion of the present invention is much more stable chemically and mechanically than conventional polymer emulsions and the polymers produced by applicants' process have an extremely small particle size making the emulsions immediately suitable for a variety of purposes.