The sorption of SeO 2- 3 on hydroxyapatite surface was investigated in batch experiments over a range of pH and SeO 2- 3 concentrations in the absence and presence of additional Ca and PO 4 . The sorption is pH dependent with a maximum observed at pH values generally encountered in natural waters. While the presence of phosphate lowers SeO 2- 3 sorption by direct competition, the presence of calcium enhances it. In order to identify the mechanism of sorption and the nature of the surface sites, microscopic observation and spectroscopic methods such as X-ray diffraction and X-ray photoelectron spectroscopy were used. Surface complexation, coprecipitation, and precipitation processes were ruled out. Localization of sorbed selenium in the crystallographic sites where phosphorus is normally located shows that selenite is sorbed on the apatite by an anionic exchange with phosphate groups. Although the exact equilibria involved could not be established, the stoichiometry of the exchange is close to 1 : 1. According to kinetics experiments and X-ray diffraction analyses, it seems that selenium is not exclusively located at the surface but diffuses slightly in a thickness of a few nanometers.