The vibrational spectra of CO adsorbed on Ru-modified Pt(100) surfaces prepared by chamical vapor deposition (comdensation of Ru-3(CO)(12) at 105 K followed by X-ray irradiation and thermal decomposition at 650 K in ultrahigh vacuum, UHV) was investigated by time-resolved infrared reflection absorption spectroscopy (IRAS) in UHV. Spectra were recorded while Ru/Pt(100) bimetallic surfaces (theta(Ru) = 0.24 and 0.52 by X-ray photoelectron spectroscopy, XPS) were dosed with gas-phase CO. Analysis of the data revealed that for a wide range of calibrated CO exposures, the linear CO-stretching region displays two features: a higher energy peak (2085-2100 cm(-1)) attributed to CO adsorbed on pristine Pt(100) sites, and a lower energy peak (2066-2092 cm(-1)), ascribed to adsorption of CO on sites on the surface induced by the presence of Ru. Similar experiments were performed on bimetallic specimens annealed repeatedly in UHV to 650 K to promote partial Ru dissolution into the lattice and thus render surfaces gradually enriched in Pt. For all surfaces and CO exposures examined, the total integrated area under the two CO spectral features remained fairly constant and equal in value to the corresponding areas found for bare Pt(100). If it is assumed that a fixed exposure leads to a fixed coverage on both bare and Ru-modified Pt(100)surfaces, and the thermal treatment leads to an exchange of Ru by Pt sites without altering significantly the total number of metal sites on the surface, the absorption cross sections for both of these peaks are virtually the same.