The potential dependence of the extended X-ray absorption fine structure (EXAFS) obtained at the Pt L-111 absorption edge for a carbon supported Pt electrocatalyst exposed to carbon monoxide is presented, The data have been analyzed using the difference file method to separate the dominant contributions of the Pt neighbors from contributions to the EXAFS from the adsorbed species. The presence of adsorbed CO is clearly observed with a Pt-C distance of 1.85 Angstrom at potentials less than 0.5 V vs RHE. Increasing the potential above 0.5 V resulted first in the removal of the adsorbed CO and at more positive potentials, e.g., 1.05 V, in the formation of an oxide layer, as evidenced by the presence of a Pt-O coordination shell at 2.00 Angstrom. These results demonstrate that in situ EXAFS of supported Pt electrocatalysts may be used to probe adsorbate structures.