Thermodynamic causes of non-stoichiometry (the so-called equilibrium non-stoichiometry) are based on the higher thermodynamic stability at T > 0 K of the non-stoichiometric composition as compared to the stoichiometric composition . Equilibrium non-stoichiometry is due to the existence of the homogeneity region, that is the temperature-concentration limits for of occurrence of some definite crystal structure of a compound. For the case of refractory compounds, their homogeneity regions have been described and classified for practically all compounds [2]. Non-equilibrium non-stoichiometry can be created in highly nonequilibrium conditions, such as super-high pressure, very high super-cooling, as well as at micro-and macro-gravity (acceleration) conditions. Thus, crystallization of stoichiometric KBr from supersaturated water solutions in the temperature range 90Β°C to Οͺ10 Β°C in a centrifuge at acceleration 6200 g results in the formation of non-stoichiometric K 1-x Br 1.07 . Non-equilibrium non-stoichiometry strongly effects the properties depending on the crystal structure. For example, micro-hardness of the K 1-x Br 1.07 crystals increased by 2.17 times as compared with the micro-hardness of stoichiometric KBr.