UNIFAC-group contribution method and the Wilson formula were used to study the solvent effects in the terminal model for radical copolymerization. Local monomer concentrations in the neighbourhood of growing polymer radicals were estimated in order to determine the true terminal reactivity ratios for the styrene-methyl methacrylate monomer system in bulk, and in benzene, toluene, benzyl alcohol and phenol. Coefficients of monomer partitioning, K-1 and K-2, in the vicinity of the growing macroradicals -M-1. and -M-2. were estimated. From the results, it was concluded that the solvent does influence the local monomer concentrations in the proximity of both growing macroradicals, -M-1. and -M-2, but not their ratios expressed as K-1(s) and K-2(s), respectively. Constants K-1(s) and K-2(s) for solution copolymerization were equal to those for bulk copolymerization K-1(b) and K-2(b), respectively. From the graphical analysis of the solvent effect, an increasing effect was suggested when changing the solvent from benzene < toluene < benzyl alcohol < phenol.