The nature of the active phase (metallic vs. oxidic, metal phase vs. concentrated hydride/ dilute solid solution with hydrogen) in heterogeneous catalysis by supported metals is still a matter of high debate. Here, we have monitored for the first time oxide-supported Pd nanocatalysts (average particle size 4.5 mn) during both CO oxidation (in H-2-free atmosphere) and preferential oxidation of CO in H-2 excess (PROX) by operand X-ray absorption spectroscopy. Under our conditions, the CO conversion in the absence of H-2 is around 30% at 150 degrees C and reaches 100% at 200 degrees C, whereas in the presence of H-2 the conversion reaches a maximum of 15% (at 250 degrees C), in agreement with our previous works using a conventional flow-fixed bed reactor. The active phase for CO oxidation below 200 degrees C is metallic Pd, whereas it is a solid solution of Pd with hydrogen during PROX below 300 degrees C. This work provides a direct evidence of the presence of subsurface/bulk hydrogen as a probable cause of the low PROX performance of supported Pd catalysts.