A series of carbon-supported nanosized platinum electrocatalysts for use in proton exchange membrane (PEM) fuel cells have been characterized electrochemically in acid solution. This analysis was performed under galvanostatic conditions, in contrast to the more traditional potentiodynamic methods that generate cyclic voltammograms (CVs). The galvanostatic method used has the advantage of collecting data more efficiently than a typical CV protocol. The resultant differential capacity curves (equivalent to a CV) for the samples analyzed showed that the electrochemical response changed as the platinum loading on a given carbon support increased. In particular, peaks developed in the low potential (hydrogen adsorption/desorption) regions while peaks corresponding to the desorption of oxygen-containing species shifted to higher potentials. A model is proposed to explain this behavior based on changes to the size of the platinum particles. (C) 2003 The Electrochemical Society.