We demonstrate that a new emerging technique, two-colour sum-frequency generation (SFG) spectroscopy, can be used to probe the molecular electronic properties of self-assembled monolayers (SAMs). In the CH spectral range (2800-3200 cm(-1)), we show that the sum-frequency generation signal of a porphyrin alkanethiol derivative adsorbed on Pt(1 1 1) reaches a maximum intensity at similar to435 nm SFG wavelength. This wavelength corresponds to the porphyrin moiety specific pi-pi* molecular electronic transition which is called the Soret or B band. This resonant behaviour is not observed for 1-dodecanethiol SAW which are devoid of molecular electronic transition in the investigated visible spectral range. (C) 2003 Elsevier Science B.V. All rights reserved.