The processes leading to degradation of Pt nanoparticles inside commercial hydrogen fuel cell catalyst layers are studied in situ, using time-resolved high-energy powder X-ray diffraction. Advances in electrochemical cell design significantly increase the quality of diffraction patterns obtained at practical catalyst loadings at high temporal resolution and allow the use of advanced techniques including differential pair distribution function analysis. Rietveld refinement of the lattice parameter and peak intensities during cyclic voltammetry or potential steps allow the separate steps of oxygen electroadsorption and place exchange in the Pt oxide growth mechanism to be clearly differentiated. The slow kinetics of the place-exchange process limit the oxide growth under standard laboratory conditions, decoupling the surface chemistry of the nanoparticles from the applied potential and directly affecting the outcome of accelerated stress tests measured with different cycling schemes. High-speed diffraction measurements are also used to follow the oxide reduction reaction, which is much faster. Refined structural parameters from these data show direct evidence for a transient, disordered platinum intermediate created during reduction of the oxide, which is likely responsible for catalyst dissolution when cycling to high potentials.