H 2 O 2 synthesis directly from H 2 and O 2 and side reactions in this system over a variety of Pd–Au/TiO 2 catalysts have been studied systemically. The effects of Pd/Au ratio on H 2 O 2 synthesis and subsequent H 2 O 2 hydrogenation were investigated. The catalysts were characterized using multiple techniques including transmission electron microscopy (TEM), in situ diffuse reflectance infrared Fourier transform spectroscopy of CO adsorption (CO-DRIFTS), X-rays photoelectron spectroscopy (XPS), and temperature-programmed desorption of H 2 /O 2 (H 2 -/O 2 -TPD). By detailed analysis of these spectroscopies and the kinetics of this reaction, new insights into various active sites of Pd upon alloying with Au were elaborated. Pd monomer surrounded by Au atoms could be the primary active site for H 2 O 2 formation. In addition, Pd ensembles were assumed to be more active for H 2 O 2 hydrogenation than monomeric Pd sites; the over-oxidation of H 2 to H 2 O likely occurred at the interface between Au and TiO 2 .