The adsorption and desorption behavior of selenite by processed Green River Formation oil shales were examined. The selenite adsorption data could be quantitatively described by both the Freundlich and Langmuir isotherms. However, greater R{sup 2} values were obtained for the Freundlich isotherms. Furthermore, selenite adsorption was not a function of the extraction process. The adsorption of selenite by the processed oil shales was irreversible. Upon dilution of the equilibrium systems, additional selenite removal from solution occurred. A thermochemical analysis of the adsorption and desorption equilibrium solutions indicated that the solutions were undersaturated with respect to a number of metal selenite solids. This indicates that precipitation processes are not influencing selenite behavior. However, not all selenite minerals were examined in the evaluation because of the lack of thermochemical data. An insufficient equilibration period for the adsorption study of the alteration of processed oil shale solids as a result of hydration reactions may also have hastened the additional removal of selenite during the desorption studies.