Ruthenium is known to improve the CO tolerance of platinum based fuel cell catalysts, but the mechanism is unclear. Some investigators believe that the main role of ruthenium is to dissociate water, and thereby provide a faster route for the oxidation of CO. Other investigators believe that ruthenium changes the electronic structure of platinum, in a way that lowers the binding energy of the CO and thereby promotes easy reaction. In this study, we have used TPD to measure the magnitude of the two effects in UHV. When we deposit 0.25 monolayers of ruthenium on Pt(110), we find that the adsorption properties of the surface change substantially. The surface only adsorbs about half as much CO and H-2. The sticking probability of water is also reduced. The beta (2) hydrogen peak disappears, whereas the alpha (1), CO peak is attenuated by a factor of 2. The binding energy of the alpha (1) CO decreases from 31 to 29 kcal/mol when ruthenium is added to the surface, whereas the binding energy of the alpha (2) CO decreases from 25 to 23 kcal/mol. The, exchange of O-18 into (H2O)-O-16 is substantially enhanced showing that the activation barrier for OH recombination is reduced from 12 kcal/mol to between 7 and 9 kcal/mol. Together these effects would be expected to produce between a 170 and 260 meV reduction in the activation barrier for CO removal from the surface. This is in close agreement with the 200 meV reduction in the potential for CO removal measured electrochemically by previous workers. Quantification of our results shows that only about 40meV of the total reduction is associated with the ligand effect, whereas the remaining portion is associated with the bifunctional mechanism. These results show that ruthenium on platinum has very different properties than ruthenium metal. It slightly weakens the CO binding, and has a more substantial effect on activating water. There also is a site blocking effect that needs to be explained.