A nonlinear method (OPT) for the estimation of the reactivity ratios in copolymerization processes is proposed. This method makes use of all experimental data (the conversion values included) and may also use the composition equations determined according to either the simple terminal kinetic model or the penultimate model. It can be extended to ternary copolymerization processes, too. Application of OPT to ternary copolymerization data lead to reactivity ratios that are different from those obtained for the respective binary copolymerizations. The former give a better fit to the experimental results than the latter.