Modifications in the electronic density of states of small Pt particles (4-5 nm diameter) dispersed in high-area carbon (XC72-R) induced by the presence of adsorbed CO were examined by in situ Pt-L(III)-edge X-ray absorption near edge structure (XANES) in a 1.0 M H3PO4 solution. An increase in the intensity of the in situ Pt-L(III)-edge XANES was observed at energies a few electronvolts positive of the white line maximum (11 567 eV), after CO was bubbled in the solution, with the electrode polarized at potentials in the range in which CO undergoes irreversible adsorption on Pt. A well-defined peak centered at 11570 eV could be clearly identified upon subtracting these spectra from those acquired in that same potential region prior to CO adsorption. This feature has been ascribed to transitions from the Pt 2p(3/2) orbital to unoccupied states above the Fermi level localized predominantly on CO.