X-ray absorption spectroscopy (XAS) is arguably the most powerful technique in detecting the dynamic profile of SACs during the reaction course, even up to 1000 K and 100 bar. In a typical XAS experiment, an atom of the measured element absorbs incident X-ray photons with energies near or above the core level binding energies of that atom. Electrons would then escape from their quantum level and carry with the excess energy as kinetic energy if the energy of Xray is larger than electronic binding energy. The corresponding XAS spectrum is acquired by recording the X-ray energy and adsorption intensity. Therefore, the change in the valance state of the active metal is reflected in the X-ray absorption near edge structure (XANES) region, while extended X-ray absorption fine structure (EXAFS) provides fruitful information on the local coordination environment in the bulk materials (such as spatial distribution, bonding conditions, and coordination numbers). Dynamic aggregation of Cu1-CeO2 SACs into Cu4 clusters during C-N coupling was monitored by operando Cu K-edge XANES spectra in Figure . to a benchmark urea yield rate of 52.84 mmol h -1 gcat -1 at -1.6 V. Likewise, the dynamic profile of dual-site NiFe SACs in oxygen evolution reaction (OER) was traced by operando XAS in Figures and. It was found that the single Ni sites promoted the key structural reconstruction into bridging Ni-O-Fe bonds. Upon increased voltages from 1.2 to 1.5 V, the rising-edge positions in the Ni K-edge XANES were positively shifted due to the higher oxidation state of Ni species in NiFe SACs. This was supported by a higher white line intensity. The reversibility with respect to changes in the oxidation state of Ni was confirmed by its partial recovery after the removal of voltage. Meanwhile, the Fe K-edge XANES only underwent a slight energy shift with increasing applied potentials, probably owing to the electrostatic interaction between catalyst and electrolyte or by OH -adsorption. Accordingly, the distance of the first coordination shell of Ni increased at higher applied potentials due to the formation of a greater extent of Ni-O coordination. Another new peak appeared at ~ 2.78 Å above 1.3 V, also reflected in the wavelet transformed (WT)-EXAFS spectra. Similarly, the local coordination of Fe centers steadily evolved at even higher potentials of > 1.5 V, indicating that the new bonds are much more difficult to form at the Fe sites than those at the Ni atoms. Such newly formed Ni-O-Fe bonds created spin channels for electron transfer, resulting in a huge improvement in the OER activity with an overpotential of 270 mV at 10 mA cm -2 . A deprotonation process to form the multiple active sites during OER and promote the O-O coupling was observed in atomic iridium stabilized on nanoporous metal phosphides for water oxidation. Dynamic evolution of Cu-N4 to Cu-N3 and further to HO-Cu-N2 via the Cu 2+ to Cu + redox cycle under ORR and CO2RR working conditions was also identified by operando XAS. The use of operando XAS for dynamic aggregations of SACs is undoubtedly crucial. However, such averaging techniques cannot differentiate local inhomogeneities (polydispersity) arising from a distribution of coordination sites on the support or structural evolution under reaction conditions. This calls for careful interpretation of experimental results. The spectroscopic acquisition time is generally several tens of minutes in conventional XAS, which may fail to capture the fast transformation process since most reactions occur rapidly and reach a steady state in a few minutes. This urges the development of time-resolved XAS techniques to uncover the chemical state evolution of working catalysts. 23, 65