The results on the structural and thermal stability of different modifications of solid solutions Bi(4)V(2-x)Fe(x)O(11) (-) (delta) (BIFEVOX), where x = 0.05-0.6, and Bi(4)V(2-x)Cu(x/2)Ti(x/2)O(11) - (delta) (BICUTIVOX), where x = 0.025-0.50, are shown. The stability is assessed by varying the thermodynamic parameters of the medium and also the time parameters with the use of modern methods, namely, the high-temperature X-ray diffraction analysis, the differential scanning calorimetry, and the dilatometry. The. gamma-modification of BIFEVOX is shown to be stable in a wide range of temperatures and oxygen partial pressures.