The influence of deviations in the substance density from the equilibrium magnitude on the phase equilibria is studied in metals and alloys. Such deviations can be associated with both the external effects and the features of the substance structure. A technique for constructing phase diagrams of the temperature-concentration-atomic-density type for alloys is proposed. The calculations involved in constructing such a phase diagram for the K-Rb system are carried out. The calculated results are in good agreement with the experimental results available.