In situ characterization of catalysts gives direct insight into the working state of the material. Here, the design and performance characteristics of a universal in situ synchrotron-compatible X-ray diffraction cell capable of operation at high temperature and high pressure, 1373 K, and 35 bar, respectively, are reported. Its performance is demonstrated by characterizing a cobalt-based catalyst used in a prototypical high-pressure catalytic reaction, the Fischer-Tropsch synthesis, using X-ray diffraction. Cobalt nanoparticles supported on silica were studied in situ during Fischer-Tropsch catalysis using syngas, H and CO, at 723 K and 20 bar. Post reaction, the Co nanoparticles were carburized at elevated pressure, demonstrating an increased rate of carburization compared with atmospheric studies.