The performance degradation is probed for a proton exchange membrane fuel cell (PEMFC) exposed to 4 ppm HCl in air at cell voltages of 0.4, 0.5 and 0.6 V. While the steady-state cell current decreases by 90% for the cells held at 0.6 V, it only decreases by 17% when the cells are at 0.4 V. We hypothesize that Cl- poisoning is more pronounced at 0.6 V vs. 0.4 V due to increase in Cl- coverage of the Pt nanoparticle electrocatalysts as the cell voltage (and electrode potential) becomes more positive, implying chemisorption coupled to an electrostatic effect. Chloride coverage increases by ca. 30% as cell voltage increases from 0.4 to 0.6 V. Further losses are caused by a 12% decrease in Pt electrochemical surface area (ECSA) due to Pt dissolution to chloroplatinate ions, followed by growth and agglomeration of Pt nanoparticles. (C) 2014 The Electrochemical Society. All rights reserved.