The Surface Acoustic Wave (SAW) phase velocity of ZnO thin films has been determined by High Resolution Brillouin Light Scattering Spectroscopy in order to Study the influence of an intermediate layer with a set of elastic constants higher than the corresponding ones for ZnO. The study of the system formed by ZnO/Si3N4/Si(100), deposited by reactive magnetron sputtering, shows a SAW phase velocity increment with respect to the ZnO/Si(100) single layer system. Results are discussed in terms of the elasticity theory, which predicts an increase of the acoustic wave phase velocity when an intermediate layer stiffer than the film at the surface is present in the system. (C) 2005 Elsevier Ltd. All rights reserved.