The activity coefficient of salt in mixed solvents was expressed as a function of VLE data of salt-containing system by using the ternary Gibbs-Duhem equation. Two criteria for testing salt data in VLE at fixed liquid composition were proposed based on thermodynamics and the common character of electrolytic activity coefficients. According to the criteria a method for testing VLE data of salt-containing system at fixed liquid composition was suggested. The quality of a data group as a whole can be evaluated by this method, and some wrong data can be indicated. The method presented in this paper is not only severe in theory, but simple and convenient in application. (C) 2004 Elsevier B.V. All rights reserved.