Thin Al films on Si(1 1 1) have been investigated by optical second-harmonic generation (SHG) in combination with synchrotron radiation photoemission spectroscopy. Valence band spectra show quantization of the Al sp-band in the energy range from the Fermi level down to the surface state at a binding energy of 4.6 eV. Quantization effects in SHG are observed as oscillations in the signal as a function of film thickness with an oscillation period that depends on the pump frequency of the laser light. The relation between film thickness and photon energy of SHG resonances for Al films indicates different types of resonances involving both quantum well and substrate states. It is shown that the structural order at the Al/Si interface can be probed through the rotationally anisotropic contribution to SHG. By exposing the film to oxygen and thus modifying the free surface, surface and interface contributions to the isotropic SH signal are identified. (C) 2003 Elsevier B.V. All rights reserved.