New invariant asymmmetric mixing rules were developed which obeyed the boundary condition of the second virial coefficient. These mixing rules were incorporated into the PRSV (Peng-Robinson-Stryjek-Vera) equation of state and applied to vapor-liquid equilibrium (VLE) calculations with emphasis on aqueous mixtures. The new mixing rules could improve VLE calculations for binary aqueous systems while an asymmetric interaction constant (l(12)) and temperature-dependent q(12) were used. The new model reasonably predicts the VLE behavior for several aqueous weak electrolyte systems and is generally better than the API-Sour model at elevated temperature applications.