An in-situ neutron diffraction experiment was performed under electrical bias on Bi4V2O11 and BICOVOX membranes in order to analyse the electrochemical reduction of these electrolytes at the cathode under working conditions. For neutrons, a large amount of material is needed and thick membranes, 8 mm in diameter, 2.5 cm in thickness, were prepared. The experiments were performed at 620 degreesC, under air atmosphere. The sample, connected to gold wires, was placed at the centre of a furnace working under vacuum and isolated from it in a quartz tube with one end opened to air. Under polansation, the bi-dimensional structure of the BIMEVOX compound is retained and the observed changes in unit-cell parameters are due to a slight variation in oxygen stoichiometry. This transformation is reversible and the initial diffraction pattern was fully recovered after the current was turned off and the membrane reheated up to 700 degreesC. (C) 2004 Elsevier B.V. All rights reserved.