Precipitation of gold by vacuum evaporation creates heterogeneity in the surface layer of GaAs, which differ in structure and composition in a direction parallel to the surface of gallium arsenide layers in the perpendicular direction. Reactions of physico-chemical interaction in the plane parallel to the GaAs surface are recorded by measuring the velocity of surface acoustic waves by virtue of the location of the velocity vectors in the wave. As it turned out, particularly informative study changes over time derivative of the velocity as a quantity proportional to viscosity. The oscillation spectrum shows the derivative of the velocity characteristics of phase transformations at the metal-semiconductor transition, including such reactions as microbreakdown, liquid phase formation and loss of islets GaAs.