A new method for estimating copolymerization reactivity ratios, by sequentially sampling the reaction medium and analyzing the samples by ultraviolet spectroscopy and gravimetry, was developed. The data were analyzed by a new error-in-variables method. The method was applied to solution copolymerization of the styrene (Sty)methyl methacrylate (MMA) system in butyl acetate at 65degreesC, for which reactivity ratios were r(MMA) = 0.660 +/- 0.050 and r(Sty) = 0.309 +/- 0.050. The data were also analyzed by the Kelen-Tudos and the Extended Kelen-Tudos methods where the methods are applicable. (C) 2004 Wiley Periodicals, Inc.