Low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and GO-temperature; programmed desorption (TPD) were used to characterize ruthenium modified Pt(100) surfaces of very high purity and controlled stoichiometry prepared in ultrahigh vacuum (UHV) by irradiating Ru-3(CO)(12) films condensed on cold Pt substrates at 150 K with X-rays, and subsequent annealing at ca. 620 K. The presence of Ru an Pt(100) lifted the (5 x 20) reconstruction characteristic of the bare clean substrate; however, the reconstruction reappeared as-the bimetallic surfaces were briefly annealed to ca. 900 K. Exposure of nonannealed Ru(theta (Ru) greater than or equal to 0.22)/Pt(100), where theta (Ru) represents the Ru coverage in monolayers, to large exposures of CO at ca. 200 K yielded smaller theta (CO), as well as TPD peaks with onset desorption temperatures, T-des(CO), ca. 50 K lower than those observed for bare Pt(100). More strikingly, however, the CO-TPD spectra of CO-saturated Ru(theta (Ru) = 0.42)/Pt(100), which had been briefly annealed to 900 K, displayed T-des(CO) as low as 250 K, very similar to desorption temperatures reported for Pt-modified Ru(0001) by de Mongeot et al.