Copolymerizations of styrene and methyl methacrylate have been performed using different monomer feed compositions in the presence of a catalytic chain transfer agent at 40°C. Average chain transfer constants as a function of monomer feed composition were determined with the conventional Mayo procedure using both number (Mn) and weight (Mw) average molecular weights, and with the chain length distribution procedure using both high (ΛH) and peak (ΛP) molecular weight slopes. It is found that the average chain transfer constants determined from Mw and ΛP are generally very similar, with those obtained from Mn and ΛH being larger and smaller, respectively. The average chain transfer constants obtained from Mw and ΛP are compared with model predictions based upon both the terminal and penultimate unit models of free-radical copolymerization and are in satisfactory agreement. These two models are used to predict the fraction of propagating radicals with a terminal styrene unit, and it is found, similar to earlier studies reported in the literature, that this parameter is very sensitive to the penultimate model s value (in contrast to the average propagation rate coefficient), and a satisfactory agreement between model and experiment is obtained for sM = 2. This finding possibly suggests a complimentary route to measuring average propagation rate coefficients, for the determination of penultimate model s values.