Temperature dependency of the charge transfer coefficient (CTC) of the oxygen electrode of a commercial proton exchange membrane electrolyzer stack is investigated in this paper. A good estimate of the CTC is important in predicting the current-voltage characteristics of electrolyzers. The estimated CTC also provides insight into the properties of the electrode. The results of this study show that at typical operating temperatures, there is a reasonable variation of the CTC from the symmetry factor. The results indicate that within a temperature range of 20-60 degrees C, the average CTC changes from 0.18 to 0.42 which is clearly lower than the standard symmetry factor of 0.5. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.