Vanadium/tungsten mixed oxide (V2O5/WO3) sensing membranes were deposited on glassy carbon substrates and used as the H+ sensor of the extended gate field effect transistor (EGFET) device. X-ray diffractograms indicated a decrease of the interplanar spacing of V2O5 after the insertion of WO3 revealing that the lamellar structure is under compressive stress. The crystallinity increases with increasing WO3 molar ratio. The film is not homogeneous, with more WO3 material sitting at the surface. This influences the response of pH sensors using the EGFET configuration. The maximum sensitivity of 68 mV pH(-1) was obtained for the sample with 5% WO3 molar ratio. For higher WO3 molar ratios, the behavior is not linear. It can be concluded that V2O5 dominates for acidic solutions while WO3 dominates for basic solutions. Therefore, the mixed oxide with low amount of WO3 is the main candidate for further use as biosensor. (C) 2010 Elsevier B.V. All rights reserved.