Au films thermally evaporated in vacuum are different from pure Au. New compounds as Au3Ga, Au7Ga2 are formed during the evaporation and "free" (non-incorporated into the crystalline lattice) Ga and As atoms appear in the Au surface layer. In this work the GaAs surface state was investigated with surface acoustic waves in situ during thermal evaporation of Au. The changes of the aggregate state caused by the Au-Ga liquid phase formation followed by the creation of the solid solutions based on the Au3Ga and the Au7Ga2 compounds are observed. Such transformations in the films seem to be determined by the diffusion of "free" Ga and As atoms towards the GaAs surface due to the plastic deformation induced by the Au evaporated flux. Data were confirmed by the precise chemical analysis and Auger electron spectroscopy. (C) 2003 Elsevier Science B.V. All rights reserved.