Changes in the state of the surface layer of semi-insulating (111) GaAs in air under the action of white light are studied using surface acoustic waves (SAWs). It is shown that, depending on the light intensity and the SAW power, the reactions of interaction with molecules, atoms, and charged particles of the air are enhanced or weakened. The role of standing acoustic waves arising in the surface layers of gallium arsenide, namely, the concentration of interaction effects in places of maximum illumination as a result of diffraction of light by standing acoustic waves, is considered. The formation of nuclei (Ga + As) in the diffraction spots, covered with layers of compounds with oxygen and carbon, either growing or spreading, is found, and the fact that the formation of an oxide layer leads to a reorientation of the GaAs surface is established.