Common classical expressions for the molar excess Gibbs energy of mixing g(E) do not contain a contribution from composition fluctuations that are significant in the vicinity of the plait point for a ternary system. We propose a correction to g(E) based on reasonable phenomenological assumptions. This correction requires three ternary constants, but two are obtained from stability relations provided that we know the composition of the plait point. While the method proposed here is not predictive, it provides a consistent thermodynamic procedure for calculating the liquid-liquid phase diagram of a ternary system with a plait point. The proposed method is illustrated for three ternaries. When calculations are based on the classical expressions for g(E) alone, calculated results are in serious disagreement with experiment near critical conditions. Inclusion of the proposed correction for g(E) provides dramatic improvement. (c) 2005 Elsevier B.V. All rights reserved.