The adsorption, desorption, and dissociation of water on the GaAs(001)-(4X2) 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)-(4X2) 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.