An exploratory study of H2S poisoning of membrane electrode assemblies (MEAs) in proton exchange membrane fuel cells (PEMFCs) consisting of Pt and Pt-Ru alloy electrodes is presented. Steady-state polarization curves are reported for each electrode after exposure to 50 ppm H2S at 70degreesC. Significant findings include (i) partial recovery of the MEA after 3.8 h of exposure to H2S; (ii) the degree of the recovery is influenced by the electrochemical oxidation of two surface species observed during cyclic voltammetry experiments; (iii) in contrast to CO poisoning, Ru has no effect on increasing MEA tolerance toward H2S poisoning; and (iv) increasing the Pt loading by 60% appears to quadruple the partially recovered current density at 0.6 V (i.e., 0.125 A/cm(2) for Pt-Ru alloy and 0.575 A/cm(2) for Pt electrodes) after exposure to neat H-2 for 24 h. (C) 2003 The Electrochemical Society.