When materials are reduced to the nanoscale, their structure and reactivity can deviate greatly from the bulk or extended surface case. Using the archetypal example of supported Pt nanoparticles (ca. 2 nm diameter, 1 wt % Pt on Al2O3) catalyzing CO oxidation to CO2 during cyclic redox operation, we show that high energy Xray total scattering, used with subsecond time resolution, can yield detailed, valuable insights into the dynamic behavior of nanoscale systems. This approach reveals how these nanoparticles respond to their environment and the nature of active sites being formed and consumed within the catalytic process. Specific insight is gained into the structure of the highly active Pt surface oxide that formed on the nanoparticles during catalysis.