We have investigated the wavelength and potential dependence of the second harmonic response from Ag(111) surfaces covered by self-assembled monolayers (SAMs) obtained from two different alpha-functionalised terthiophenes in comparison with bare surfaces in the spectral region where interband transitions can occur. We have analysed both isotropic and anisotropic components of the second harmonic signal at different directions of the polarisation. A strong dependence on the potential, the wavelength and the nature of the film has been observed. Our results are consistent with those of Bradley et al. for bare surfaces and give additional evidence that the enhancement of the second harmonic process observed is mainly caused by a resonance with some elements of the surface susceptibility tensor. (C) 2005 Elsevier Ltd. All rights reserved.