A GaAs surface chemically treated with hydrochloric (HCl) acid was characterized by measurement of the contact angle between an HCl-treated GaAs surface and a water droplet, measurement of the Hall mobility of a Ge film deposited on the surface, and by Auger electron spectroscopy. As the temperature during the HCl treatment increased the contact angle of a water droplet increased, indicating that the hydrophobicity of GaAs surface changed. Auger electron spectroscopic analysis revealed that an HCl treatment prevents GaAs surface oxidation without formation of a detectable arsenic layer on the surface. The Hall mobility of a Ge film deposited on an HCl-treated GaAs substrate is higher than those deposited on as-etched GaAs substrate due to the removal of the oxide layer on the substrate.