The reduction of Bi4V2O11 and BIMEVOX powders was investigated using two methods: thermogravimetric analyses and high-temperature X-ray diffraction under hydrogen flow. The chemical reduction, due to a V-V-V-IV reduction, was compared to the evolution of cathodes of dense BIMEVOX membranes under current bias which had previously been characterised using in situ X-ray synchrotron diffraction. This correlation enables to quantify the level of reduction reached on Bi4V2O11, membranes under operating conditions. This explains the oxygen transfer in the materials, at least at the cathode under bias. (C) 2003 Elsevier Science B.V All rights reserved.