In order to extend the validity of existing mechanistic polymerization models, it is necessary to conduct experiments beyond typical temperature ranges. Bulk copolymerizations of butyl acrylate/methyl methacrylate were performed over an expanded temperature range (60 to 140 degrees C) compared to most studies published to date which have focused on a more limited temperature range (60 to 80 degrees C). From the large amount of collected data, reactivity ratios were estimated at five different temperatures. The reactivity ratios were estimated from low conversion copolymer composition data using the error-in-variables model method (EVM). The low conversion data were subsequently coupled with high conversion data and the reactivity ratios were reestimated using non-linear parameter estimation based on the integrated copolymer composition equation. Using all of the available data, temperature-dependent expressions were developed for the reactivity ratios. The copolymer composition data conformed to the Mayo-Lewis kinetic model over the entire temperature range and departures from this model due to depropagation reactions, a concern at elevated temperatures, were not observed.