SynopsisIn this article the straight-line relationship between In A and In B, where A and B denote the concentrations of the two types of monomers in any reaction period, is examined to exist in the radical copolymerizations of 1.02 2 rIrz 2 0.25. Utilizing this observation, we propose an easy-touse compositions calculation method in which a single empirical parameter is included. The method is derived without involving the constant monomer reactivity ratios assumption. The copolymerizations of styrene-methyl methacrylate, vinyl chloride-vinyl acetate, methyl methacrylate-vinyl acetate, acrylic acid-acrylamide, methacrylic acid-methacrylamide, and sodium methacrylate-methacrylamide are investigated. The instantaneous and cumulative copolymer compositions and the residued monomer compositions computed by this proposal are in very good agreement with the experimental data.