Hydrogen ion diffusion in ruthenium oxide film is of significant interest because of its importance in capacitor, sensor and catalyst applications. In this study, a method based on potential-pH response measurement was used to determine hydrogen ion diffusion in ruthenium oxide films. The drift in the potential-pH response is believed to be due to the hydrated layer, which affects hydrogen ion diffusion onto the oxide film of the pH sensor. Hydrogen ion diffusion coefficient of ruthenium oxide films obtained from this method was 6 x 10(-14) cm(2)/s. The unique feature of the potential-pH response method is its relatively simple experimental procedure, which eliminates complications arising from surface related effects and/or presence of hydrogen traps in membrane such as those found in the conventional permeation method. (C) 2002 Elsevier Science B.V. All rights reserved.