Experimental and theoretical efforts toward the unde standing of the chemical properties of heterometallic junctions are reviewed. The main focus is b ought onto the valence electronic structure of interfaces and its relevance to the adsorption of carbon monoxide. Examples such as Cu/Ru (001), Fe/Ru (001), Pd/Ta (110), Pd/Al (111) and Pd/Au (111) are presented and models describing dominant interaction in these systems are outlined to discuss the nature and origin of unique properties of the heterometallic junction.