The mode of bonding of CO to Ru-modified Pt(100) surfaces prepared by chemical vapor deposition was investigated using infrared reflection absorption spectroscopy (IRAS) in ultrahigh vacuum (UHV). Spectra recorded in the CO stretching region for freshly prepared Ru(theta(Ru) ca. 0.52)/Pt(100) exposed to saturation coverage of CO displayed, in addition to a rather weak peak ascribed to CO bound to bridging sites, a single asymmetric peak centered at ca. 2092 cm(-1). Statistical analyses of IRAS spectra recorded in experiments in which a Ru(theta(Ru) ca. 0.52)/Pt(100) surface was annealed sequentially to 650 K for ca. 2.5 h prior to a saturation exposure of CO yielded two peaks at 2100 cm(-1) and 2092 cm(-1) with a clearly identifiable isosbestic point, characteristic of a quantitative conversion of one type of species into the other. This behavior has been attributed to a thermally induced surface incorporation of Ru into Pt, as has been shown with low-energy ion-scattering spectroscopy (LEIS) for other Ru-modified low-index single-crystal Pt surfaces (Davies et al. Electrochim. Acta 1998, 44, 1181), which effectively enriches the surface with Pt while reducing the Ru coverage.