Non-linear curve-fitting regression method using parametric equation as 'user model' has been utilized to determine reactivity ratios signifying effects of terminal and penultimate groups in copolymerization. Residuals, standard errors and correlation coefficients have been compared with those of terminal model using Fukuda's data on styrene-methyl methacrylate copolymerization. Non-uniqueness and occurrence of two sets of values for the penultimate model reactivity ratios have been explained on the basis of specified feed composition calculations. The two sets of reactivity ratios corresponding to explicit penultimate model have been used for treatment of kinetic data and consistent and acceptable values of various kinetic constants have been obtained with only one set. The value of phi obtained corresponds very closely to theoretically predicted value. (c) 2005 Elsevier Ltd. All rights reserved.