Changes in the Pt L-III-edge region of small Pt particles (4 to 5 nm diam) dispersed in a high-area carbon (XC72-R) induced by the adsorption of CO, were monitored in situ in aqueous 0.1 M HClO4 by measuring the fluorescence intensity at fixed x-ray energies during a voltammetric cycle. This real-time technique made it possible to detect a small increase in the normalized fluorescence (<0.1) at the Pt L-III main absorption edge (11,564 eV), following electro-oxidation of CO adsorbed at saturation coverages on the high-area Pt, derived from a shift in the edge toward lower energies. These in situ findings are in agreement with ere situ spectroscopic measurements of Pt surface core levels for CO adsorbed on Pt single crystals in ultrahigh-vacuum environments. Also described in this work are applications of this new in situ technique to the study of hydrogen adsorption-desorption and oxide formation and reduction on such high-area Pt/XC72-R electrodes in the same electrolyte.