For understanding the structure and function of catalysts in action, researchers of Karlsruhe Institute of Technology (KIT), in cooperation with colleagues from the Swiss Light Source SLS of Paul Scherrer Institute (PSI) in Switzerland and the European Synchrotron Radiation Facility (ESRF) in France, have developed a new diagnostic tool. Operando X-ray spectroscopy visualizes the structure and gradients of complex technical catalysts in three dimensions, thus allowing us to look into functioning chemical reactors. The results are reported in Nature Catalysis.