An a priori analysis of the Wilson and NRTL equations is carried out. The possibilities whether these equations are suitable for the simultaneous description of the excess Gibbs energy GEand the excess enthalpy HEare investigated. It follows from this analysis that the Wilson equation for strongly non-ideal solutions is not capable of quantitative describing simultaneously the functions GEand HE. The applicability of the NRTL equation is restricted by its impossibility of changing the sign of the excess entropy in the permissible concentration region (0, 1).