The numerical integration method was developed to determine the relative reactivity ratios according to the simplified penultimate model (systems with r(2) = 0). It allows one to calculate the copolymer composition for zero conversion level. This procedure was used to verify the Walling-Briggs method. For this purpose literature data for the monomer system styrene/methyl alpha-cyanocinnamate were evaluated in terms of both methods. On increasing the mole fraction of M-2 in the feed ([M-2] > 0.60) the deviations of the Walling-Briggs method from the numerical procedure increased, reaching 1%-3% even at low conversion levels. This agrees with the theoretical considerations. The calculated copolymer composition for the zero conversion obtained from the numerical integration method was further used to determine the relative reactivity ratios by means of the linear Kelen-Tudos method. Up to 23 wt.% conversion, no substantial differences were found between the obtained parameters using the copolymer compositions determined in terms of both integration methods. (C) 1997 Elsevier Science Ltd.