The temperature dependence of the reactivity ratios of the system styrene/butyl acrylate is studied. Copolymers of styrene and butyl acrylate are therefore synthesised in bulk at 50, 90, 100 and 120 degrees C. Copolymer composition is determined by H-1 NMR, the resulting data are evaluated, and Arrhenius parameters are obtained for r(BA) In order to investigate the molecular-weight dependence of the copolymer composition, copolymerisations of styrene and butyl acrylate are carried out at 50 and 90 degrees C in the presence of mercapto ethanol. No molecular-weight dependence is found for the investigated system.