The interaction of vapor-deposited Al with the Ru(0001) surface and its influence upon CO chemisorption has been studied in ultrahigh vacuum by use of Auger electron spectroscopy, thermal desorption mass spectroscopy, and low-energy electron diffraction (LEED). Below 670 K, the Al grows as an overlayer in (111) oriented islands. By 1170 K, Al penetrates the surface to form intermetallic compounds with the Ru, and a (root 3 x root 3)R30/sup 0/ LEED pattern dominates the surface structure. This is interpreted in terms of a stoichiometric monolayer of the compound Al/sub 2/Ru. Thermal desorption data indicate that this monolayer alloy has a heat of formation of about -24 kcal/mol, compared to -14 kcal/mol for the multilayer alloy which nucleates at higher coverages. The CO/Ru interaction at the surface of these intermetallic compounds is not very different from that on the clean Ru(0001) surface, except for the expected dilution of Ru sites by the added Al. In contrast, when the Al overlayer has not yet been annealed and the Al adatoms still sit on top of the Ru(0001) surface, they have a very strong influence upon CO desorption spectra. A new CO desorption state at 850-1050 K appears at submonolayer Al coverages,more » which is attributed to strong, direct, interaction between adsorbed CO and Al probably involving C-O bond cleavage.« less