Methods for calculation of liquid-vapour phase transitions frontiers in metal systems at atmospheric and low pressures are offered. Possibility of the system’s components separation process forecasting by their evaporation is shown on examples of thallium—lead, copper—cadmium, cadmium—zinc, lead—antimony phase diagrams. Influence of pressure decrease on change of phase transition frontiers position is considered. It is shown that liquid—vapour phase transitions in vacuum allow estimate value of energy consumption decrease in consequence of solutions boiling temperature decrease and possibility of primary evaporation of one of metals. Determination of vapour phase’s equilibrium composition and, therefore, condensate in distillation process, and also minimum admissible depression below which take place a precipitation of firm phase in the rest of distillation is possible according to the shape of solutions and steam coexistence area.