In situ X-ray absorption spectroscopy (XAS) measurements, including both X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) at the Pt L-3 and Ru K edges, were carried out on three different carbon-supported PtRu electrocatalysts in an electrochemical cell in 1 M HClO4 with 0.3 M methanol. The CO and OH adsorbate coverage on Pt and Ru were determined as a function of the applied potential via the novel delta XANES technique, and the particle morphology was determined from the EXAFS and a modeling technique. Both the bifunctional and direct CO oxidation mechanisms, the latter enhanced by electronic ligand effects, were evident for all three electrocatalysts; however, the dominant mechanism depended critically on the particle size and morphology. Both the Ru island size and overall cluster size had a very large effect on the CO oxidation mechanism and activation of water, with the bifunctional mechanism dominating for more monodispersed Ru islands, and the direct surface ligand effect dominating in the presence of larger Ru islands. (c) 2007 The Electrochemical Society.