Some new methods for verification of vapour-liquid equilibrium data are described. These methods are based on stability conditions and so have a strict thermodynamic foundation. Equations for the calculation of the limiting values of the equilibrium thermodynamic variables for multicomponent mixtures (possible extreme values of boiling temperature, vapour pressure, phase compositions, etc.) from the data for the constituent mixtures (sub-systems) are set out. Agreement between limiting values and experimental data is shown. In some cases (for heteroazeotropes) the results of the computations of the limiting values are for practical purposes the calculation of actual azeotropic compositions.