The structure refinement of La-doped Bi2V0.90Cu0.10O5.5-delta (BICUVOX.10) showed that the maximum solid solubility of lanthanum cations in the bismuth sublattice corresponds to approximately 2-2.5% of the Bi site concentration. Doping with lanthanum leads to a decrease in both oxygen ionic and electronic conductivities of Bi2-xLaxV0.9Cu0.1O5.5-delta (X = 0-0.20). The oxygen ion transference numbers determined by the modified e.m.f. method under the oxygen pressure gradient of 101/21 kPa at 820-970 K vary in the range 0.962-0.999, increasing with the decrease in temperature. Reduction of the oxygen partial pressure down to 10 kPa leads to a drastic increase of the n-type electronic conductivity and then to phase decomposition. In contrast to the instability of Bi2V0.90Cu0.10O5.5-delta at intermediate oxygen partial pressures, annealing at high oxygen pressures (up to 12 MPa) results in neither phase decomposition nor considerable increase of the p-type electronic transport. (C) 2002 Elsevier Science B.V. All rights reserved.