The molecular orientation of carbon monoxide adsorbed on Pd(111) was examined by sum frequency generation (SFG) vibrational spectroscopy utilizing different polarization combinations of the visible and SFG light. This allows to determine the CO tilt angle with respect to the substrate, provided that a proper optical model for the interface can be defined. It is demonstrated that it is essential to invoke the beta(aac) hyperpolarizability into the analysis and that polarization-dependent SFG of CO/Pd(111) yields information on beta(acc)/beta(ccc) rather than the tilt angle. (C) 2002 Elsevier Science B.V. All rights reserved.