Pulsed-laser polymerisation has been used to investigate the radical copolymerisation kinetics of styrene with methyl ethacrylate. Experiments were conducted over the temperature range 30-80 degreesC. The results for copolymer composition and the average propagation rate coefficients, [k(p)], can be interpreted using terminal model kinetics. It is argued that the primary factor influencing the copolymerisation [k(p)] data is the sterically hindered methyl ethacrylate radicals and this steric effect swamps any enthalpic penultimate unit effect which may be expected to play a role in the copolymerisation reaction. The terminal model reactivity ratios are a strong function of temperature and this reflects the importance of depropagation reactions at higher temperatures and lower concentrations of methyl ethacrylate.