The binding energies and momentum distribution of the four valence orbitals of CO have been investigated by high-resolution electron momentum spectroscopy. Experimental momentum profiles are compared on a quantitative basis to theoretical calculations using SCF wavefunctions ranging in quality from minimal basis to a recently published 136-GTO wavefunction which is essentially at the Hartree-Fock limit. Calculated momentum distributions for the 5σ orbital are very basis set dependent while calculations of DZ or better quality give very similar momentum distributions for the respective 3σ, 4σ and Dr orbitals. The 1π experimental momentum profile is not well described in the low-momentum region even at the Hartree-Fock limit with basis set saturation including diffuse functions. Binding energy spectra and momentum profiles in the satellite region beyond 22 eV binding energy of CO are studied in detail. Measurements in the binding energy spectrum at 24 and 28 eV have experimental momentum profiles which are assigned to the 4σ and 1π main lines respectively. The structure above 30 eV is shown to be predominantly due to satellites of the 3σ orbital. © 1988.