Deposition of thin Au films with thermal evaporation in vacuum is widely used in the technology of the GaAs microwave and optical devices. The film formation in fact is a multi-stage process, including various physical and chemical transformations, that is interpreted as thin-phase epitaxial growth. It was studied by many authors, mostly by observation of the results of metal deposition with optical or electron microscopy, Auger spectroscopy and other methods. In this paper we introduce results of in situ monitoring of the metal (Au) deposition process with surface acoustic waves. SAWs at frequencies near 100MHz were exited at the (100) surface of the semiinsolating GaAs with interdigital transducers. Phase velocity and attenuation of the SAW was precisely measured and analyzed as a function of the film deposition time (film thickness). Specific time-domain responses and results of Fourier analysis give information about surface and near-surface transformations during the deposition and give new opportunities to control this process [1-2]. SAW diagnostics can be applied to monitoring and control of any epitaxial growth processes, if SAW propagation in the substrate is provided.