The adsorption, desorption, and dissociation of water on the GaAs(001)-(4×2) surface have been studied using Auger electron spectroscopy (AES), temperature-programmed desorption, and high-resolution electron energy loss spectroscopy. We have found that water first adsorbs molecularly at 100 K and dissociates readily upon annealing by virtue of overlapping desorption and dissociation temperatures between 150 and 200 K. The dissociation probability of water on the GaAs(001)-(4×2) surface is approximately 0.8 at low coverages (exposures below 0.5 L). However, the decomposition products of water exhibit a high recombination probability, making the oxidation of GaAs difficult. A large fraction of surface hydroxyls are rehydrogenated to produce desorbing water at temperatures between 300 and 700 K. Hence, we have applied a cycling treatment (repeated adsorption of water at 100 K followed by annealing to 750 K) in order to effectively oxidize the GaAs surface. During cycling, we have monitored GaAs–oxide growth using AES. In addition, thermal desorption spectra recorded after exposure of the cycling-treated GaAs surface to water at 100 K point to molecular adsorption and intact desorption of water with little evidence of dissociation, which suggests that the surface has been significantly oxidized by the cycling treatment of water. © 1998 American Vacuum Society.