The formation of self-assembled monolayers (SAMs) of alkanethiols on GaAs was studied using attenuated total reflection Fourier transform infrared spectroscopy. SAMs formed from an ethanol solution containing ammonium hydroxide were more ordered and stable than those obtained by the method previously reported in which surfaces were derivatized from neat molten alkanethiols at elevated temperatures. It is suggested that ammonium hydroxide etches the native surface oxide of GaAs during an initial step followed by the chemisorption of alkanethiols on the chemically reactive surface. The effect of ammonium hydroxide concentrations on the SAM coverage was investigated. Well-ordered layers with close packing were formed when the ammonium hydroxide concentration was 3 vol %. SAMs of alkanethiols with different chain lengths showed that the order increases with increasing chain length (from C12H25SH to C18H37SH).