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, H2 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.