The saturation pressures and vapor–liquid compositions of 7 mixtures of the system acetone–benzene have been determined from a temperature of 100 °C and a pressure of about 2 atm up to the highest temperatures and pressures at which liquid and vapor coexist. The critical temperatures were determined by the disappearance-of-meniscus method.The P–T–X relations at the liquid–vapor phase boundaries, as obtained by the determination of the bubble-point pressure vs. temperature curves of a series of mixtures of known composition, do not indicate the existence of an azeotrope in the range of temperature and pressure of this research. The binary data have been treated thermodynamically to yield the liquid-phase activity coefficients. The partial molal volumes in the liquid mixture required for the Poynting correction (effect of pressure on liquid phase properties) for liquid-phase activity coefficients have also been obtained. The fugacity coefficient of a component in the vapor mixture has been obtained by a modified Redlich–Kwong equation, as suggested by Chueh and Prausnitz. Following their modification of the van Laar equation, several binary liquid phase parameters, such as the binary interaction constant, Henry's constant and dilation constant, as required for the solution model for excess Gibbs energy, have been calculated.