This paper presents an analysis of transient current measurements on an electrochromic device in order to evaluate the performance of the ion conductor and the average ion mobility in the device. A proton conducting all-thin-film device with WO3 and hydrated NiO as the working and counter electrodes, respectively, and ZrO2 as the ion conductor was studied. Potential reversal experiments were performed, and the resulting transient currents were measured and analyzed. The average proton diffusion coefficient for the whole device was found to be of the order of 10(-14) cm(2)/s and the ZrO2 layer was shown to be the rate-limiting layer. (C) 2005 The Electrochemical Society. All rights reserved.