Stable chemical engineering of stoichiometric GaAs [100] surfaces was achieved by deposition of two types of mercaptobiphenyls: 4'-methyl-4-mercaptobiphenyl and 4'-hydroxy-4-mercaptobiphenyl, which can render the surface hydrophobic and hydrophilic, respectively. Topography of the engineered surface was studied by atomic force microscopy (AFM), and the covalent binding between the thiolate and surface arsenide was confirmed by high-resolution X-ray photoelectron spectroscopy (HRXPS). Total surface free energies of the engineered surfaces as well as its dispersive and polar components were calculated from contact angle measurements. Electrochemical properties of the engineered GaAs in aqueous electrolytes were measured by impedance spectroscopy at a cathodic potential (-350 mV), demonstrating that both types of mercaptobiphenyls can form stable monolayers with high electric resistances, R > 2 MOmega cm(2). The surface engineering method established here allows for control of surface free energies toward deposition of model biomembranes on GaAs-based device surfaces.