Solution copolymerizations of butyl acrylate/methyl methacrylate in toluene were performed over an expanded temperature range (60-140 degrees C) compared to more typical ranges that do not exceed 80 degrees C. From a large amount of data collected independently at two laboratories, reactivity ratios were estimated at five different temperatures. The reactivity ratios were estimated from low conversion copolymer composition data using both the error-in-variables model method and a nonlinear parameter estimation based on the integrated copolymer composition equation. Using all of the available data, temperature-dependent expressions were developed for the reactivity ratios and compared to previously published bulk copolymerization values. No significant differences appeared to exist between the bulk and solution polymerization reactivity ratios. Furthermore, the copolymer composition data conformed to the Mayo-Lewis kinetic model over the entire temperature range.