UNIFAC-group contribution method for determination of local monomer concentrations and the simplex method for determination of reactivity ratios were used to describe the terminal and penultimate models of copolymerization in the styrene-acrylonitrile monomer system in bulk at 333.15 K. Results show that the procedure is applicable for prediction of the kinetic behaviour of the comonomer pair. Local monomer concentrations, true reactivity ratios, distribution coefficients K and the newly introduced parameter A, measuring deviation of the local concentration from the global concentration, were calculated. According to the results, the absolute value of the distribution coefficient K depends on the type of reactivity ratio but not on the monomer ratio in the feed.