Measurements of the heats of mixing of gases at pressures near to atmospheric have been used to obtain second virial cross-coefficients B-12 for mixtures of (cyclohexane + methanol or ether or acetone) and of (benzene + methanol or ether or acetone). Given pair potential parameters for the pure components and appropriate combining rules, the heats of mixing and values of B-12 for each of the cyclohexane mixtures can be predicted with good accuracy. For the mixtures containing benzene this is not true as the measured heats of mixing are less endothermic than the calculated values. The difference is attributed to specific interactions which take place between benzene and the polar molecules. The difference can be analysed in term of a simple quasi-chemical theory which yields values of the enthalpy of association Delta H. When this was done the values of Delta H were found to be -12.3 kJ mol(-1) for water + benzene, -14.7 kJ mol(-1) for methanol + benzene, -5 kJ mol(-1) for ethyl ether + benzene, and -17 kJ mol(-1) for acetone + benzene. These values are in addition to energy of interaction due to dispersion forces.