The polymerization of vinyl monomer initiated by an aqueous solution of sodium polystyrenesulfonate (PSS-Na) was carried out at 85°C. Methyl methacrylate (MMA) and styrene were polymerized, while acrylonitrile was not. The rate of polymerization of MMA decreased with the increase of the degree of polymerization of PSS-Na. However, the polymerization was not initiated by sodium ethyl benzenesulfonate which was a unit molecule of PSS-Na. The polymerization proved to be a radical reaction. The polymerization was considered to commence with the formation of hydrophobic areas with PSS-Na in the aqueous phase. MMA is incorporated into these areas, and there the polymerization is initiated and proceeds. The hydrophobic areas were assumed to be similar to the micelles formed by anionic detergents such as sodium alkylbenzene sulfonate. An initiation mechanism is proposed.