The total vapour pressure and the composition of the liquid and vapour phases of the benzene and n-heptane system have been experimentally determined under equilibrium conditions at 110, 125, 140, 155, 170, 185, 200 and 215 "C. The corresponding composition of the vapour phase was also estimated using an expression for the liquid phase activity coefficient as a function of the composition of the liquid phase. Several expressions were compared and it has been found that the three-constant Redlich and Kister equation gave the best fit of the experimental vapour pressure data than the other equations.Comparison between the calculated and measured vapour compositions has shown a discrepancy of ymeas. -ycalc. of the order of 0.02 mol fraction or 10 times the experimental tolerance. Nevertheless, the experimental values were reasonably thermodynamically consistent.The low values of the liquid phase activity coefficients indicated that the behaviour of this particular system was not far from ideality. Also, it was found that the degree of non-ideality decreased with the increase in temperature. The behaviour of the vapour phase was shown also to be nearly ideal.The excess Gibbs free energy of mixing was calculated, and it was found to be non-symmetrical with mole fraction. On increasing the temperature of the system, the liquid composition corresponding to the maximum GE tended to shift to a higher benzene concentration, whilst at a fixed liquid composition, GE initially decreased and then remained stationary or increased again. This was thought to be due to differences in the molar volumes of the two components. 113?