Monomer reactivity ratios in free radical copolymerization of styrene and butyl acrylate monomers at 80 degrees C in toluene solution under reduced pressure were determined for low and high conversions. Mayo-Lewis, Finemann-Ross, inverted Finemann-Ross, Kelen-Tudos, extended Kelen-Tudos, Tidwell-Mortimer and Mao-Huglin methods for low conversion and extended Kelen-Tudos and Mao-Huglin methods for high conversion were used to calculate r(S) and r(B) in this paper. Using the Mao-Huglin method, for conversions below 15%, monomer reactivity ratio values of 0.887 and 0.216 were calculated for styrene and butyl acrylate, respectively. For conversions greater then 15%, these values were found to be 1.006 and 0.232, respectively. In addition, the reactivity ratios calculated from the second-order Markov reaction were compared with that of calculated using the first-order Markov reaction. (C) 1997 Elsevier Science Ltd.