A mixing rule is developed to incorporate solution model in equation of state (EOS) to extend the EOS to highly nonideal mixtures. To obtain the mixture parameter a of the EOS, the excess free energy of the solution model is matched to that of the EOS at a molar volume equal to a fixed multiple of the excluded volume b such that all of the mixed liquids are at a low pressure (by the EOS). The mixing rule is applicable to cubic EOS as well. as to noncubic perturbation type EOS, for which the zero-pressure-based or infinite pressure-based mixing rules are not applicable. A wealth of solution model parameters that have been fitted to low-pressure data are made useful by means of the mixing rule for phase equilibrium calculations at elevated pressures. Examples are presented of the use of the UNIFAC model with previously reported parameters and the Wang-Chao-Wilson model using newly fitted parameters incorporated into the Peng-Robinson EOS and the chain-of-rotators group contribution EOS.