A direct method for evaluating equilibrium vapor compositions from measurements of the vapor pressure or boiling point of the solutions is presented. The basic differential equations are derived from Gibbs-Duhem equation and can be solved easily by iterative method. This method can be successfully used for binary systems

and in principle can also be applied to multicomponent systems. Several examples of binaries and ternaries are presented for demonstration. Calculated results are in good agreement with experimental data.