AbstractThe augmented BACK equation of state for polar fluids is extended to mixtures by introducing mixing rules separately for the nonpolar and polar interactions. Fluid phase equilibria are calculated for polar + nonpolar, and polar + polar mixtures including mixtures of hydrogen‐bonding substances. Enthalpy calculations are illustrated with enthalpy of H2S + N2 and heats of mixing of steam mixtures, and carbon dioxide mixtures.Cross interaction coefficients are adjusted for the correlation of data on binary mixtures. Both phase equilibrium and enthalpy data are correlated at the same time with two such coefficients for each binary. The cross interaction coefficients are constants, invariant with temperature, density, or phase state.