In this paper, we describe the deposition of octadecylthiol (ODT) monolayers on highly doped n-GaAs electrode surfaces, which showed high stability both in air and in aqueous electrolytes. In the first part of this study, four different wet chemical etching procedures were investigated to optimize surface treatment before ODT deposition. The chemical composition of the surface was evaluated by X-ray photoelectron spectroscopy (XPS), demonstrating that the photochemical etching procedure (called "etch P" in this study) can generate a surface enriched with arsenides, which can serve as the binding sites for sulfides. In the second part of this study, the surface prepared by etch P was coated with an ODT monolayer. The monolayer showed high stability in air, as indicated by the constant ellipsometric thickness. In electrolytes, the stability of the monolayer-coated surface was verified by impedance spectroscopy under zero-current potential (U-j=0 = -360 mV) for more than 10 h; then the stability of the interface was monitored under different bias potentials. Electrochemical passivation of the GaAs surface has been demonstrated for the first time under physiological conditions tin aqueous electrolyte, near neutral pH), which allows for the application of GaAs electrodes to biological systems.