Ruthenium from a carbon supported platinum-ruthenium anode catalyst can be deposited in the membrane, cathode catalyst layer, and microporous layer (MPL) on the cathode side of a polymer-electrolyte fuel cell. The presence of Ru in the MPL signals the invasion of the MPL by liquid water. Little Ru was observed in the MPL of a cell potentiostated at 0.7 V. Conversely, a significant amount of Ru deposited in the MPL of a cell potentiostated at 1.2 V, indicating that liquid water can enter an oxidized MPL. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.028210jes] All rights reserved.