The monomer system methyl methacrylate (MMA = M(1))/N-vinylpyrrolidone (NVP = M(2)) was copolymerized at 60 degrees C with azobis(isobutyronitrile) as the initiator in cyclohexane (c-H), methyl isobutyrate (MiB), and N-methylpyrrolidone (NMP) at the total monomer concentration 0.4 (mol . L(-1)). The relative reactivity ratios ri were estimated by the nonlinear least-squares curve fitting by Tidwell and Mortimer, and they depend on the nature of the solvent. For the description of this effect the reactant-solvent complex (RSC) model was taken into consideration. Since the analytical copolymer composition equation derived is inconvenient for very small ri values, the Monte Carlo method was used. It permits us to describe all tendencies correctly in terms of the RSC model. In the present paper the RSC model was tested for the first time in application to a binary copolymerization, whereby preevaluated relative reactivity parameters were used instead of fitting the model to experimental data. In this manner a check of the physical meaning of the model increments was achieved. The highest deviation between measured and calculated copolymer compositions amounted to 3-5 mol%. Solvents especially affect the resonance parameter and of the monomers which changes 4-fold by a change from c-H to NMP. The polarity parameter e remains almost unchanged. Observed effects can be understood in terms of the molecular orbital (MO) energies of the reacting partners.