The electro-oxidation of carbon monoxide and methanol on platinum-ruthenium bulk alloys was investigated by cyclic voltammetry and in situ FTIR reflectance spectroscopy. Electrochemical measurements revealed that these two oxidation reactions are dependent on the ruthenium composition in the alloy. For CO oxidation, the alloy with an Ru concentration of 50 at.% allows this reaction to occur at lower potentials than on pure platinum or pure ruthenium. Conversely, methanol electro-oxidation gives higher current densities on alloys, containing 10-15 at.% Ru. With FTIRS measurements, it was found that, during methanol oxidation on Pt-Ru alloys, CO, but also small amounts of HCHO and HCOOH, are formed, the surface distribution of which depend on the alloy composition. The COads absorption bands are quite absent in spectra corresponding to alloys with the optimum composition (15-20 at.% Ru). On the other hand, during the oxidation of gaseous CO on alloys with the optimum composition (Pt0.5Ru0.5), the CO2 formed is not adsorbed on the electrode surface, the opposite of the other compositions. These results suggest strongly that the rate determining step for both reactions is the formation of adsorbed OH on the ruthenium surface.