Molecular-orbital energy shifts are observed in photoemission from weakly physisorbed CO on clean and Xe-covered Al(111) surfaces. These shifts in ionization potentials are mainly due to final-state relaxation effects, which can be described approximately by a point-charge image-potential model. Differential distance- and orbital-dependent energy shifts suggest that CO molecules lie flat on the substrates. CO is adsorbed on Al(111) with a heat of formation of 0.21 eV/molecule. © 1980, All rights reserved.