An analysis of X-ray absorption spectroscopy (XAS) data [X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS)] at the Pt L-3 edge for Pt-M bimetallic materials (M = Co, Cr, Ni, Fe) and at the Co K edge for Pt-Co is reported for Pt-M/C electrodes in HClO4 at different potentials. The XANES data are analyzed using the Delta mu method, which utilizes the spectrum at some potential V minus that at 0.54 V reversible hydrogen electrode (RHE) representing a reference spectrum. These Delta mu data provide direct spectroscopic evidence for the inhibition of OH chemisorption on the cluster surface in the Pt-M. This OH chemisorption, decreasing in the direction Pt > Pt-Ni > Pt-Co > Pt-Fe > Pt-Cr, is directly correlated with the previously reported fuel cell performance (electrocatalytic activities) of these bimetallics, confirming the role of OH poisoning of Pt sites in fuel cells. EXAFS analysis shows that the prepared clusters studied have different morphologies, the Pt-Ni and Pt-Co clusters were more homogeneous with M atoms at the surface, while the Pt-Fe and Pt-Cr clusters had a "Pt skin." The cluster morphology determines which previously proposed OH inhibition mechanism dominates, the electronic mechanism in the presence of the Pt skin, or lateral interactions when M-OH groups exist on the surface. (c) 2005 The Electrochemical Society.