The nature of the active phase (metallic vs. oxidic, metal phase vs. concentrated hydride/ diluted solid solution with hydrogen) in heterogeneous catalysis by supported metals is still a matter of high debate.1 Here, we have monitored for the first time oxide-supported Pd nanocatalysts (average particle size 4.5 nm) during both CO oxidation (in H2-free atmosphere) and preferential oxidation of CO in H2 excess (PROX) by operando X-ray absorption spectroscopy (Rock beamline at Soleil). Under our conditions, the CO conversion in the absence of H2 is around 30% at 150 °C and reaches 100% at 200 °C, whereas in the presence of H2 the conversion reaches a maximum of 15% (at 250 °C), in agreement with our previous works using a conventional flow-fixed bed reactor.2 The active phase for CO oxidation below 200 °C is metallic Pd, whereas it is a solid solution of Pd with hydrogen during PROX below 300 °C. This presentation will clarify the role of hydrogen in supported Pd catalysts during the PROX reaction.3 1. H.A. Aleksandrov, S.M. Kozlov, S. Schauermann, G.N. Vayssilov, K.M. Neyman, Angew. Chem. Int. Ed. 53, 13371–13375 (2014) 2. C. Zlotea, F. Morfin, T.S. Nguyen, N.T. Nguyen, J. Nelayah, C. Ricolleau, M. Latroche, L. Piccolo, Nanoscale 6, 9955–9959 (2014) 3. C. Zlotea, Y. Oumellal, K. Provost, F. Morfin, L.Piccolo, Applied Catalysis B: Environmental 237, 1059-1065 (2018)