AbstractThe desirability of adopting some common method of plotting vapour‐liquid equilibrium data on systems with more than one liquid phase is stressed and attention is drawn to the lack of agreement in published literature on the theoretical treatment of such systems. The conditions under which both the Gibbs‐Duhem equation and the expression connecting the excess free energy of mixing with the activity coefficients apply to systems with more than one liquid phase are critically examined. As a result of this theoretical study a method of plotting equilibrium data on systems with two or more liquid phases is recommended and thermodynamic tests for the consistency of experimental data are presented. The method of plotting experimental results and applying tests for consistency are illustrated by an analysis of the published data on the systemn‐butanol