We have investigated the time dependent conductance of a WO3 film (similar to 700 K) after a voltage step. From the beginning to some 100 ins, the region next to the positive electrode shows a strong increase in resistance. The region next to the negative electrode shows a decrease in resistance. A possible explanation for this first effect is the depletion (respectively accumulation) of mobile donors. After 10 s, a decrease in resistance next to the positive electrode is observed. The region next to the negative electrode shows an increase in resistance. The resistance of the two regions nearly meets its value prior to the perturbation. A possible explanation for this second effect is desorption (respectively adsorption) of charged surface oxygen. (c) 2005 Published by Elsevier B.V.