The surface electrochemistry of irreversibly adsorbed bismuth (Bi-ir) on the Pt(111) electrode in 0.1 M HClO4 has been studied by in situ surface X-ray scattering to correlate the atomic structure with the voltammetric features. Bi-ir saturates at a coverage of similar to one-third of a monolayer and forms a disordered structure on the Pt(111) surface with some evidence that the Bi-ir is mobile. If the Bi-ir coverage is slightly increased, then there is a reduction in the charge under the reversible peaks in the cyclic voltammetry at a potential of similar to0.68 V (vs Pd/H-2). The charge under the reversible peaks is most likely associated with the adsorption of oxygenated species at platinum sites adjacent to Bi-ir atoms. Saturation of the electrolyte with CO and subsequent potential cycling leads to displacement of Bi-ir from the surface and the appearance of an X-ray diffraction pattern due to a p(2 x 2)-3CO adlayer that has previously been observed on the unmodified Pt(111) electrode.