The flux of permeate in the separation by the direct-contact membrane distillation process as a function of membrane characteristics and operating conditions was considered. The expressions describing the vapor transfer through a hydrophobic membrane, the temperature at the surface of the membrane, and changes of temperature and concentration solutions along the module in the countercurrent device were derived and proposed. The theoretically predicted results show about 85-90% agreement with the experimental data. The utility of equations in the separation technology was pointed out.