Although the heat of infinitesimal vaporization should properly be called the heat of vaporization for mixtures, its measurement or calculation has been very difficult. In the present work, practical and direct expressions of the heat of vaporization for mixtures are proposed newly. These expressions have their roots in well-known thermodynamic relations. They provide examples for the calculation of the heat of vaporization of mixtures, which are useful, since such examples are scarce in the literature. Any type of equation of state is applicable to these new expressions. The expressions for a closed system are related to infinitesimal vaporization at constant pressure or constant temperature, and the expression for a continuous process represents a liquid-vapor interface where steady vaporization and condensation are taking place. Calculations of heat of vaporization using the new expressions are demonstrated for propane-isobutane mixture. This demonstration highlights the fact that the heat of vaporization of mixtures is quite dependent upon the process, system, and composition. (c) 2005 Elsevier Ltd. All rights reserved.