The determination of the individual conversion rate of each monomer in a radical copolymerization allows the absolute concentration of each kind of growing radical to be derived, provided the reactivity ratios and the homopropagation rate constants are known. The procedure is applied to the following binary copolymerizations: styrene-methyl methacrylate, vinyl chloride-vinyl acetate, acrylonitrile-vinyl acetate, and acrylonitrile-vinyl chloride. In the two latter cases, the anomalous behavior, probably caused by the internal cyclization reaction involving CN bonds, is well evidenced.