Abstract The unoccupied band structure of a CO monolayer on Ni(110), Pd(110), and Pt(110), respectively, has been investigated by inverse photoemission. For Ni and Pd the CO-derived band structures exhibit only marginal differences. The observed bands are interpreted in terms of 2π x - and 2π y -derived COCO bonding and antibonding bands. The level ordering in the center of the surface Brillouin zone, as obtained from a polarization analysis, revealed a similar splitting for the 2π x and the 2π y levels at normal incidence. This indicates a COCO interaction of approximately the same magnitude in the x as well as the y direction. Consequently, the CO molecules seem to be arranged in a nearly hexagonal structure at monolayer coverage which in turn is due to the COCO repulsion dominating over the CO-substrate interaction in the tightly packed monolayer. The observed level ordering of the 2π y bands is consistent with such a geometry. On Pt(110), a distinctly different electronic structure indicates a different bonding of the CO to the metal substrate, namely a reduced 2π backbonding and a more prominent role of the 5σ dative bonding. This may also give rise to a slightly different adsorption geometry.