New mixing rules which incorporate lattice fluid excess functions (for example Wilson and NRTL) and the correct quadratic concentration dependence of the second virial coefficient are presented. It is shown that the excess Gibbs funct:ions derived for activity coefficient models correspond to the attractive part of the excess Helmholtz energy derived from EOS. This allows the formulation of a class of mixing rules which are independent of the repulsive term in the EOS, and are applicable to both cubic and noncubic EOS. The mixing rules are applied to different low-pressure phase equilibria and to two near-critical and supercritical systems.