Investigation of non-stoichiometry of important classes of inorganic materials is often hampered by a very narrow (< 0.1 at.%) range of existence of solid phases. A new approach to this problem based on experimental study of P-T-X (pressure-temperature-composition) phase equilibrium is discussed. From the vapor-pressure measurements, the space arrangement of the single-phase volumes of the conjugated condensed and vapor phases is reconstructed from a data set of scanning points. It has been shown that standard vapor-pressure equipment makes it possible to directly determine the composition of the condensed phase with an accuracy of up to (10(-3)-10(-4)) at.% at high temperature, thus allowing studies of crystals with a sub-0.1 at.% non-stoichiometry range. Results of vapor-pressure scanning of high-T-c superconductor and semiconductor materials are presented.