The usual copolymerization equation obtained for the terminal charge transfer model by Seiner and Litt is discussed. A new definition, valuable especially for K-values higher than 0.03 and for systems having low probabilities of the homopropagation reactions, is proposed for the reactivity ratios. The higher generality of the new equation is demonstrated on two systems. Two FORTRAN programs for nonlinear estimation with least-square error of the parameters are presented and discussed.