A new method has been developed for determining the number of gas-phase and liquid-phase transfer units from the tray efficiency measured under distillation conditions. This method, which accounts for the effect of physical properties on both mass-transfer coefficients and interfacial area, is different from the other three methods that have been previously published in the literature. In this study, the new method and the other three methods were compared, using tray efficiency data from six binary mixtures that have a wide range of physical properties. The results show that the new method yields the most consistent results and that the other three methods are not always valid. The restrictions for the application of the latter methods have been identified.