A new method is proposed for controlling the composition (nonstoichiometry) of low-volatile inorganic compounds. The basic principle of the method is the introduction (or removal) of one of the components into the low-volatile compound using reversible selective chemical vapor transport (CVT). Theoretical analysis is used to identify the process parameters determining the direction of mass transport: source temperature T 1 , source composition x 1 , and sample temperature T 2 . Using nonequilibrium thermodynamics concepts, steadystate conditions are found under which mass transport ceases. A new type of phase diagram, x 2 -T 1 -T 2 , is proposed, which describes the state of CVT systems under steady-state conditions without mass transport. The CVT process is used to prepare GaSe crystals with different deviations from stoichiometry. The crystals are characterized using x-ray diffraction and cathodoluminescence spectroscopy. The stability regions of two GaSe polytypes in the T -x phase diagram are located. CVT is used to control the compositions of phases in the In-S system. The advantages of the CVT method are analyzed with application to control over the composition of inorganic compounds. S102 INORGANIC MATERIALS Vol. 40 Suppl. 2 2004 ZAVRAZHNOV et al .