A corresponding states approach has been applied to the two-constant equations of state by Wilson, Soave, Peng—Robinson, Hamam et al., Lu et al., Simonet—Behar, and Chaudron et al. in order to obtain the equivalent shape-factor correlations. The correlations derived are compared with the Leach—Leland corresponding-states theory. Different fluid approximations for mixtures have been applied to the various corresponding states approaches. The resulting computation methods have been applied to calculate saturation properties of pure fluids and separation factors in binary mixtures for some fluids commonly encountered in natural gas and petroleum refining operations. Finally it is shown that the binary interaction parameters depend only on the fluid approximation and not on the specific form of the potential.