The physical significance of terms in two excess Gibbs free energy (G(ex))-based mixing rules, the modified Huron-Vidal (MHV1) and Wong-Sandler (WS) mixing rule, are examined through the use of solvation free energy. It is found that these mixing rules are in fact matching the charging contributions of solvation in an equation of state (EOS) to the complete solvation free energy in a liquid activity coefficient model (LM). The cavity contributions in the EOS are canceled as a result of the constant liquid molar volume to molecular volume ratio. The underlying idea of G(ex)-based mixing rules that the EOS should behave like a LM at some limiting condition breaks down due to such an improper matching of solvation free energy components. (c) 2008 Elsevier B.V. All rights reserved.