The mechanism of H2O adsorption on GaAs(100) has been elucidated by an adaptation of the photoreflectance (PR) technique for surface kinetic measurements. Being an optical method, PR is especially well-suited for probing weakly bonded adsorption systems where the pressures required for significant interaction (≳10−5 Torr) preclude the use of traditional electron or ion spectroscopies. H2O adsorbs through a physisorbed state. This species can desorb or react to a chemisorbed form, which in turn can desorb. Both the physisorbed and chemisorbed species are undissociated. We interpret exceptionally low values for the prefactors associated with the chemisorbed state in terms of an adsorbate-induced surface reconstruction.