Six equations of state are compared for their ability to correlate, at high pressures and temperatures, the residual enthalpies of carbon dioxide, n-hexane, and toluene, and the enthalpies of mixing for the three binary mixtures of those compounds. The correlations were used to predict the ternary mixing enthalpies Equations of state were chosen that have from two to six adjustable parameters. Only one adjustable interaction-parameter was used for the binary mixtures. No adjustable parameters were used for the ternary mixture predictions. Most of the predictions of the best equation of state have standard deviations for each of the ten temperatures (T) and pressures (P) of measurement of 8% or less of the absolute maximum enthalpy of mixing at each T and P.