The infinite dilution activity coefficients (gamma(i)(infinity)) of a solute in a solvent are important data in process separation calculations. These values reflect the degree of non-ideal solution behavior of the solute in the solvent. This paper investigates the use of infinite dilution activity coefficients in cubic equation of state mixing rules for the prediction of phase behavior at high pressures. A mixing rule recently developed by Twu and Goon has been extended from infinite pressure to zero pressure. The methodology for extending the infinite-pressure Twu-Coon mixing rule was developed so that the zero-pressure Twu-Coon mixing rule reproduces the excess Gibbs free energy, as well as liquid activity coefficients of any activity models, with extremely high accuracy without requiring any additional binary interaction parameters. We compare the performance of this new mixing rule with the MHV1 and Wong-Sandler mixing rules for its ability to use gamma(i)(infinity) in the prediction of high pressure phase behavior for strongly non-ideal systems. (C) 1998 Elsevier Science B.V. All rights reserved.