An analytical approach to predict copolymer compositions is presented for the particular case that the reactivity of one monomer (B) alters under the influence of one measurable medium parameter. Terpolymerization mathematical treatment was applied to binary systems (A/B), which turn into ternary systems (M-1, M-2, M-3) in a defined range between two extremes of the medium parameter. The approach needs as input data a relationship describing the coexistence of two derivatives (M-2, M-3) of B as a function of the influencing parameter, knowledge of homopolymerization kinetic data of B, and reactivity ratios, r(A) and r(B), only at the extremes of the variable medium parameter. As a case study, the copolymerization of acrylamide/acrylic acid mixtures in aqueous medium was selected with the pH as variable parameter. Good agreement of predicted and experimental data was proved. The approach provides a useful tool to significantly decrease the number of polymerization experiments needed to identify the effect of medium parameters on the polymerization kinetics. The principle of the approach is proposed to treat any set of two monomers if one alters the characteristics under the influence of any measurable medium parameter.