Vapour - liquid equilibrium data for the binary system alpha-pinene + beta-pinene were determined at 53.3 and 80 kPa in a modified Othmer still. Using these data and the data published by Tucker and Hawkins (1954), parameters for the Wilson, NRTL and UNIQUAC equations were calculated with the aid of a Nelder - Mead technique to minimize an objective function based on the total pressure. The results were compared with those obtained by correlation of individual isobars. Predictions at 373.15 and 388.15 K, at which Reich and Sanhueza (1992) had obtained vapour - liquid equilibrium data, were made. The UNIFAC model was also used to predict data for the isobars and the isotherms studied.