Vibrationally resolved C and O 1s photoelectron spectra of carbon monoxide have been measured in the photon energy range between 300 and 440 eV and between 550 and 702 eV, respectively. The intensity ratios of the vibrational components vary slowly, approaching the sudden limit. From the spectra in the sudden lit-nit, the information about the potential curves for the C 1s and O 1s ionized states are extracted. The experimental potential curves thus obtained are reproduced well by the present ab initio calculations based on the symmetry adapted cluster-confliguration interaction method. (c) 2005 Elsevier B.V. All rights reserved.