AlN-based compounds are studied as potential candidate to be used as substrate in surface acoustic wave (SAW) devices. In this paper, we present a study of the acoustic waves phase velocity behavior of a Al-x Ga1-x N thin film set prepared by molecular beam epitaxy on Si(111) (0 less than or equal to x less than or equal to 1 covering the full x-range). This study has been carried out by Brillouin spectroscopy and shows a nearly linear compositional dependence in the full range of concentrations. The maximum value the phase velocity for waves traveling along the film plane directions (11 200 m s(-1)) is reached for AlN. (C) 2002 Elsevier Science B.V. All rights reserved.