As a factor that may affect the accuracy of the measurement of electrode potentials in proton-exchange membrane fuel cells, the potential difference in a strip of Nafion (R) 117 membrane induced by a humidity or temperature gradient was examined. The sample was placed in a cell with two chambers, where the temperature and humidity could be controlled individually. Hydrogen gas was fed to both chambers, and the voltage between two electrodes set at the ends of the sample, which acts as a reversible hydrogen electrode, was measured. A higher potential of the electrode was observed when humidity or temperature was set lower than the other electrode. (C) 2009 Elsevier B.V. All rights reserved.