SACs using flame spray pyrolysis only contain the Pd 2+ oxidation state, independent of the CO oxidation reaction conditions. The agglomeration of Ir atoms on Fe3O4 at 960 K during CO oxidation and water-gas shift reaction was validated by in situ XPS. The dynamic evolution of CuO during the reduction/oxidation treatments in deNOx reactions was also traced by in situ XPS. Complete reduction of CuO to metallic Cu occurred in H2 reduction, leading to the shift of the typical Cu 2+ peak at 934.0 eV to a lower binding energy. The recovery in the XPS profile to its initial state after re-oxidation in wet air confirmed good reversibility. The excellent stability of Ni1Cu2/g-C3N4 catalyst in hydrogenation could be verified by in situ XPS, where the Cu 2p3/2 binding energy was invariant at 933.2 eV for the +1 oxidation state, irrespective of the treatments. Nevertheless, most NAP-XPS studies are conducted in the quasi-operando conditions in gas-phase, calling for the development of adapting the XPS technique into or close to practical reaction conditions, particularly for electrochemical or liquid-phase reactions.