Ey measuring the (vapour + liquid) equilibrium of { methanol (1) + benzene (2) + NaI} and testing the data using the ternary Gibbs-Duhem equation, the experimental results of the (vapour + liquid) equilibrium with thermodynamic consistency are obtained. It is supposcd that the mean activity coefficients of NaI in (methanol + benzene) mixed solvents may be represented by a power series of salt concentration (m(1/2)). Each parameter of the series was then obtained from the experimental results by the method of least squares. The calculated results show that the activity coefficients of NaI in (methanol + benzene) system with constant composition either decrease as the concentration increases, or decrease at first, then pass through a minimum and increase gradually again. This method is applicable to the determination of electrolytic activity coefficients in mixed non-aqueous solvents. (c) 2006 Elsevier Ltd. All rights reserved.