New procedures are developed for use with the recent Wong-Sandler mixing rules for the prediction of mixture phase equlibria from cubic equations of state. Improvements in the apriori evaluation of both the liquid-phase parameter A(infinity)(E) and the gas-phase parameter B-12(EOS) (or K-12) are made. An important new procedure is developed where the latter parameter is found from experimental measurements of the cross second virial coefficient B-12(Exp) in the single-phase vapor mixture at low pressures. Apriori prediction of phase eqilibria to high pressures and temperatures then follows when the liquid-phase parameter A(infinity)(E) is estimated from a group-contribution method, such as UNIFAC. An order-of-magnitude analysis shows the resultant phase equlibria to be more sensitive to the gas-phase parameter B-12(EOS). These new procedures are most promising in the prediction (not correlation) of the phase behavior of complex systems, such as CO2/H2O/oil in petroleum reservoirs undergoing CO2 and/or steam flooding.