We report results from coulometric measurements of the oxide coverage on dispersed platinum ͑Pt͒ 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°C, 100% relative humidity͒. The Pt oxide is thus limited to that formed from H 2 O 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 ,

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... g 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.