The ability of cuprite (Cu2O) to reduce the mobility of selenite species (SeO32- or HSeO3-), by means of chemical immobilization from an aqueous medium, was evaluated from batch experiments. The effect of pH was critically discussed as it was found to have a major influence on the nature of the mechanism involved in the uptake process of selenite by cuprite. Three pH-dependent mechanisms were evidenced by combining quantitative analysis of selenite solutions after equilibration with cuprite, solid-phase characterization, and surface analyses. In the presence of protons, the immobilization of selenite occurs via the formation of a copper selenite solid (i.e.,'chalcomenite, CuSeO(3)(.)2H(2)O). The driving force is the acid-catalyzed dissolution of cuprite into Cu-I species, followed by their fast disproportionation into CuO and Cu-II, this latter reacting with selenite to give the precipitation product. In an alkaline medium (pH > 7.5), selenite was mainly accumulated on the cuprite surface according to two distinct mechanisms depending on the time scale of the experiment. A fast adsorption process was predominant at short equilibration times (typically a few minutes), involving the substitution of the surface hydroxyl groups of Cu2O by the selenium oxyanion. Longer times (several days) led to the-progressive coverage of the cuprite surface by transient Cu(OH)(2), which was found to react with HSeO3-species to form an insoluble layer on the cuprite particles. Furthermore, in relation to the possible use of cuprite as a scavenger for radioactive selenium species in polluted environments, the competitive influence of chloride anions on the immobilization processes was investigated and discussed with respect to the kind of uptake mechanisms.