A new copolymerization equation is suggested for the calculation of copolymer composition. The new equation is independent of reactivity ratios, so we can obtain the copolymer composition from experimental data when the reactivity ratios are unknown. The new equation has been verified by experimental data (styrene/methyl methacrylate, methyl methacrylate/vinyl chloride, and styrene/acrylonitrile systems). The results of the comparisons suggest that the new equation is satisfactory.