Tungsten trioxide (WO3) films are used extensively in eletrochromic devices [1], surface acoustic wave gas sensors [2] and photo-catalytic degradation [3]. In recent years, there has also been a growing interest in WO3-based ceramics. Due to their high dielectric constant, WO3 ceramics have been proposed for applications in ceramic capacitors [4]. WO3-based semiconducting ceramics have been successfully used for detection and control of toxic gases. In 1994, Makarov et al. reported nonlinear current–voltage characteristics of Na2CO3 and MnO2-doped WO3 ceramics [5]. Now much progress has been achieved in preparing WO3-based ceramic varistors with low breakdown voltage [6]. Their high dielectric constant enables them to act as a varistor in parallel with a capacitor, which is attractive for applications in elimination of electrical noise of micro-motors, protecting contact of delays and absorbing discharges of some circuits. For practical applications, it is important for materials to exhibit stable physical properties in the course of time. Unfortunately, the electrical properties of sintered WO3 ceramics have been found quite instable. When a constant DC voltage is applied on a WO3 ceramic sample, the current responded very slowly and its magnitude would decay even by a factor of 10 with increasing time [7]. It is a great challenge to improve the stability of the physical properties of WO3 ceramics and there have been extensive researches on it. In ZnO-based ceramic varistors and Pb(Zr,Ti)O3 (PZT) thin film capacitors, the reduction of hydrogen from the electrolysis of water has been recognized as an important origin for degradation of their electrical properties [8, 9]. Presently, we have studied the influence of hydrogen on WO3 ceramics. Our results show that hydrogen greatly changes the properties of WO3 ceramics and much attention should be paid to it in WO3-based components and devices. Analytical grade (‡99%) WO3 powder was ballmilled with de-ionized water for 24 h. After drying, 1 wt% PVA was added as binder and the powder was pressed into pellets of 16 mm in diameter and 2 mm thick. The pressed pellets were sintered at 1150 C for 2 h. Silver electrodes of 4 mm in diameter were fired onto the centers of the two major surfaces of the pellets. Some pellets were placed in a 0.01 M NaOH solution and DC voltages were applied between their silver electrodes and a counter Pt electrode in the solution. The applied DC voltages induced electrolysis of water and the silver electrodes of the pellets acted as the cathode with hydrogen deposited on them. This treatment is referred to as ‘‘electrochemical hydrogen charging’’ hereafter. The DC voltages were removed after some designated periods of time and the pellets W. P. Chen (&) X. F. Zhu Z. J. Shen J. Q. Sun J. Shi Department of Physics and Key Laboratory of Acoustic and Photonic Materials and Devices of Ministry of Education, Wuhan University, Wuhan 430072, P.R. China e-mail: chenwp66@yahoo.com