The molecular orientation in self-assembled monolayers, of thiophenol (TP) on Ag(1 1 1) and of para-nitroanilino-dodecane-thiol (NAT) on Au was investigated by infrared-visible sum-frequency generation between 500 and 1500 cm(-1). Ab initio calculations were performed to determine the interface nonlinear response. The combined experimental and theoretical results reveal that the S-C bond of TP is tilted 37degrees from the Ag(1 1 1) surface normal, with the aromatic ring plane perpendicular to the surface. On Au, the tilt angle of the NAT molecule stabilizes below 60degrees at the end of film growth. (C) 2004 Elsevier B.V. All rights reserved.