The activity towards CO and methanol electrooxidation of bare and platinum-modified Ru(100) surfaces has been investigated. The structure/morphology and composition of the modified surfaces were characterized using electron diffraction techniques (LEED, RHEED) and Auger spectroscopy. The bare Ru(100) surface exhibits a higher catalytic activity towards CO electrooxidation than the Ru(0001) surface due to the lower oxidation potential of the former surface. The early stages of surface oxidation lead to disordering of the surface and further enhancing of the electrocatalytic activity. Electrodeposition of Pt on Ru(100) leads to epitaxial growth via a Volmer–Weber growth mode. The Pt clusters grow preferentially with the (311) plane parallel to the substrate surface with (01) rows in the layers in contact with the substrate compressed by about 3% with respect to bulk Pt, in order to match with the (120) rows of the Ru(100) surface. This compression leads to enhanced catalytic activity towards CO oxidation for thin Pt deposits whereas for large deposited Pt particles the dominating factor for the catalytic enhancement is the higher concentration of surface defects. On the other hand, in the case of methanol oxidation, the dominant factor in determining the catalytic activity is the concentration of adjacent Pt–Ru sites, although surface defects play an important role in the methanol dehydrogenation steps.