We have studied the interaction of CO with epitaxial graphene on Pt(1 1 1) and Ru(0 0 0 1) by means of high-resolution electron energy loss spectroscopy measurements. Our experiments unambiguously demonstrate that in ultra-high vacuum conditions CO does not intercalate underneath the graphene monolayer supported on Pt(1 1 1) and Ru(0 0 0 1). For submonolayer coverages of graphene on Pt(1 1 1), CO adsorption occurs only in Pt on-top sites while bridge sites, usually populated on the clean Pt(1 1 1) surface, are inhibited. Instead, we find that epitaxial graphene is rather reactive toward water molecules.