During a condensed-phase transition at equilibrium in a vaporization experiment, three phases are present. In this paper, equations relating vapor pressure, temperature, and compositions of the vapor and condensed phases are derived for systems undergoing such transitions. Previously observed unusual phenomena, such as vapor pressures that increase at constant temperature and vapor pressures that increase with decreasing temperature, are explained. It is shown that equilibrium condensed-phase transitions in the presence of the vapor are always hysteretic in the temperature; the transition occurs at a higher temperature in the increasing-temperature direction than in the decreasing-temperature direction. The particular cases of effusion and transpiration experiments are treated in detail.