An electrochemical technique using thin Pt wires as working electrodes was employed to determine the concentration of peroxide within the proton exchange membrane (PEM) of operating fuel cells. The existence of hydrogen peroxide was clearly observed, and its concentration depended primarily on membrane thickness, with thinner membranes displaying higher peroxide concentrations. H2O2 is most likely formed on the anode side of the cell through reduction of O-2. © 2005 The Electrochemical Society. [DOI: 10.1149/1.1904988] All rights reserved.