Experimental and theoretical investigations of the C-O stretch vibration of CO on a stepped platinum surface (Pt(5 3 3)) reveal distinct differences between spectra obtained with reflection absorption infrared spectroscopy (RAIRS) and sum-frequency generation (SFG). The SFG data reveal large variations in the C-O stretch intensity with increasing coverage, whereas the RAIRS data show a continuous increase. The data can be quantitatively reproduced using a model that accounts for the different dependence of SFG and RAIRS on the dipole-dipole coupling. (c) 2005 Elsevier B.V. All rights reserved.