An analysis of the mixing rules proposed by Wong and Sandler (1992) for the attractive and covolume parameter of a cubic equation of state is presented. This set of mixing rules (WS MR) reproduces the correct quadratic composition dependence of the second virial coefficient and, in addition, retains the ability to predict directly high pressure-high temperature phase equilibria from existing low-pressure GE models. It is demonstrated that the proposed mixing rules are valid for symmetric systems, but not for asymmetric ones, where the basic assumptions behind them do not hold anymore.