A proton exchange membrane fuel cell has been designed to enable correlation of cathode catalytic activity with structural parameters determined by in situ XAS investigations in transmission mode. The cell design is described and validated for oxygen reduction activity measurement using both Pt and PtCo alloy catalysts. Findings from in situ XAS investigations at the Pt L III edge are presented for a 60% Pt/C cathode catalyst operating at OCV, under load with oxygen feed, and during time resolved potential step experiments with oxygen and nitrogen feeds. In nitrogen XANES and EXAFS analysis show increasing oxide formation and particle disruption with increasing potential up to 1.0 V. In oxygen the catalyst structure remained unchanged irrespective of the applied potential and therefore load drawn from the cell. Differences in coordination number at the same applied potential in oxygen compared to nitrogen indicate that a particle morphology change occurs during the ORR.