For the first time, we perform in-situ X-Ray Absorption Spectroscopy on both Au L-3 and Pd K edges during the direct synthesis of hydrogen peroxide in a high-pressure gas-liquid-solid microreactor using SiO2 supported Pd, Au and AuPd catalysts under relevant reaction conditions. The catalysts were coated on the walls of 320 mu m ID micro capillaries, used in this work both as microreactors and as spectroscopic cells, ensuring safety of operation, ease of portability and absence of reactor effects during the measurements. XANES analysis revealed that the fresh Pd and AuPd catalysts contained a mix of Pd/PdO phases which was readily reduced in H-2 atmosphere. The presence of only metallic Pd under reaction conditions for both catalysts proved that metallic Pd is the active phase for this reaction. Au was present in its metallic form in both Au and AuPd under all conditions tested. Unlike the AuPd, monometallic Pd showed an increase in the Pd-Pd distance under reaction conditions, attributed to the formation of Pd-hydride. We propose that the absence of Pd-hydride in the AuPd catalysts reduces further hydrogenation of H2O2 to water, thereby explaining the increased selectivity to H2O2 observed with AuPd catalysts. (C) 2019 The Authors. Published by Elsevier Inc.