Conditions are considered under which the crystal lattice of nonstoichiometric compounds loses stability as structural vacancies form. The lattice is shown to become unstable when an infinite vacancy cluster constituted by vacancies of least admissible size arises in it. A method is proposed which permits the calculation of the critical vacancy concentration corresponding to the lower boundary of the homogeneity region for nonstoichiometric compounds. As an example, the lower boundaries of the homogeneity regions are calculated for nonstoichiometric transition metal carbides.