Carbon monoxide oxidation on bare and Pt-modified ruthenium surfaces with the (10 (1) over bar0) and (0001) orientations was investigated with cyclic voltammetry, scanning-tunneling microscopy and in situ Fourier transform infrared spectroscopy. Facile oxidation kinetics of CO on Ru(10 (1) over bar0) are observed, in contrast with a slow reaction on Ru(0001). Scanning tunneling microscopy (STM) measurements confirmed that spontaneous deposition of Pt produces island-like structures on both single crystal Ru surfaces. CO oxidation on a bimetallic Pt/Ru(10 (1) over bar0) surface with a Pt coverage of approximately one monolayer occurs at potentials that are 140 mV more negative than those for bare Pt. This potential is, however, more positive than the potential of the onset of the oxidation on Ru(1010). IR spectroscopy shows one peak for linearly adsorbed CO on bare and Pt-modified Ru(10 (1) over bar0) surfaces, while two peaks are visible for the Pt-modified Ru(0001) structure. A single broad peak for the bimetallic Pt/Ru(10 (1) over bar0) surface may result from addition of the red-shifted peak for Pt and the peak for the Ru(10 (1) over bar0) substrate. A large red shift of CO on the Pt/Ru(10 (1) over bar0) surface requires further work to be explained. A negative shift of CO oxidation on Pt/Ru(10 (1) over bar0) indicates a decrease of the Pt-CO bond strength on that surface compared with the bond with bulk Pt.