A formula dependent on two parameters has been proposed for the approximation of typical phase-equilibrium functions. This formula can be used for analysis of a number of mass-transfer processes, in particular, distillation, rectification, adsorption, etc. Using the dependence proposed, a mathematical model of periodic distillation of a binary mixture in the presence of a dephlegmator has been analyzed; in the dephlegmator, partial condensation of the separated mixture occurs in a nonstationary mode under certain supplementary conditions. Analytical expressions describing this process and convenient for calculations have been obtained.