A GaAs surface treated with pure water was characterized by using contact angle measurement, Auger electron spectroscopy, and atomic force microscopy. The contact angle was found to be dependent on treatment conditions such as treatment time, agitation of pure water, dissolved oxygen concentration in pure water, and illumination. The contact angle of a water droplet on GaAs surface treated with agitated deionized pure water was found to increase with increasing treatment time, indicating that the hydrophobicity of the surface increased. The agitation of pure water strongly influenced the oxygen concentration and the roughness of the treated GaAs surface, and hence, the contact angle of a water droplet on it.