The authors have studied the dependence of the composition of a copolymer on M or intrinsic viscosity on radical copolymerization of styrene with acrylonitrile and methacrylic acid. The composition of the copolymer is found to depend on the concentration of radical initiator or chain transfer agent (RSH, CBr 4, substituted silicon hydrides) as demonstrated by direct experiments on fractionation of the copolymers of initial conversion, from which it follows that the latter differ fundamentally in composition. The general nature of the effect is demonstrated in experiments on the influence of the concentration of the radical initiator on the composition of the copolymer and the magnitudes r 1 and r 2. The dependence is found on homogeneous copolymerization of styrene with methacrylic acid, acrylonitrile, vinyl acetate, vinylidene chloride; vinyl acetate with vinylidene chloride and 2-methyl-5-vinyl pyridine; and methylmethacrylate with vinylidene chloride.