Supported Pd, Au, and Pd-Au alloy catalysts are characterized with in situ diffuse reflectance infrared Fourier transform spectroscopy of CO adsorption (DRIFTS), quantitative powder X-ray diffraction, and X-ray photoelectron spectroscopy. The spectroscopic results presented in the paper demonstrate the existence of electron density transfer between Pd and Au atoms in alloy surfaces. In particular, the modification of the Pd electronic structure by the addition of Au is confirmed probably for the first time by the DRIFT spectra. The relationship between surface composition and catalyst performance in the synthesis of hydrogen peroxide directly from hydrogen and oxygen was established. Preliminary results indicate that the activity and selectivity of Pd-Au alloy catalysts can be significantly enhanced through adjusting the surface structures by changing the Au content in alloys.