We report results from coulometric measurements of the oxide coverage on dispersed platinum (Pt) as a function of potential in an O-2-free but otherwise normal H-2/air fuel cell cathode environment (0.6-0.9 V-RHE, 80 degrees C, 100% relative humidity). The Pt oxide is thus limited to that formed from H2O and provides a lower bound of the oxide present in an operating fuel cell in which O-2 is present. We found that the oxide coverage does not depend on previous exposure to gas-phase O-2, indicating that the Pt oxide formation reactions are reversible within the experimental parameter space. The results are in excellent agreement with a previously published fundamental model. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3257595] All rights reserved.