Vinyl polymerization of acrylonitrile initiated by the Mn3+/ascorbic acid redox system was investigated in aqueous sulfuric acid in the temperature range of 30–45°C. The rate of polymerization Rpand the rate of Mn3+ion disappearance were measured. The effect of additives like water -miscible organic solvents, neutral salts, complexing agents and different surfactants on the initial rate of polymerization were investigated. Depending on the results obtained, a suitable reaction mechanism is suggested which involves the formation of a complex between Mn3+ion and the acid whose decomposition generates the initiating free radical, with the polymer chain being terminated by mutual combination of the growing chains.