Oxygen deficiency of Bi2V0.90Cu0.10O5.5-delta (BICUVOX.10) solid electrolyte was studied by the coulometric titration technique and thermogravimetric analysis at oxygen partial pressures from 1 x 10(-7) to 0.5 atm (atmospheric air) in the temperature range 650-1050 K. Within the phase stability domain, the nonstoichiometry (6) varies in the narrow range from 0.150 to 0.155. Increasing oxygen deficiency leads to a greater n-type electronic conductivity, which can be described by common models for other solid electrolytes. The partial molar enthalpy and entropy for oxygen incorporation into Bi2V0.9Cu0.1O5.5-delta lattice linearly decrease with increasing delta. Further reduction of the oxygen partial pressure results in decomposition of Bi2V0.90Cu0.10O5.5-delta, forming a mixture of an Aurivillius-type phase and binary metal oxides, which is accompanied with decreasing ionic conductivity. The results of the coulometric titration and ion transference number measurements suggest that BICUVOX. 10 ceramics can be used as electrolyte only at atmospheric or higher oxygen pressures, preferably at temperatures below 900-950 K.