In this work, we have Studied the sorption of selenium (Se-79 is one of the main radionuclides in a spent nuclear fuel repository) on magnetite (Fe3O4), a mineral present in the near-field of a nuclear waste repository that might represent an important retardation factor for the mobility of many radionuclides. The sorption of both Se(IV) and Se(VI) onto magnetite has been fitted by a non-competitive Langmuir isotherm with Gamma(max) (3.13 +/- 0.07) x 10(-6) mol m(-2) and K-L = (1.19 +/- 0.07) x 10(6) dm(3) mol(-1) for Se(IV) and Gamma(max) = (3.5 +/- 0.2) x 10(-6) mol m(-2) and K-L = (3.0 +/- 0.1) x 105 dm(3) mol m(-1) and K-L = (3.0 +/- 0.1) x 10(5) dm(3) mol(-1) for Se(VI). The variation of the sorption of selenium with pH has been modeled using the Triple Layer Surface Complexation Model and the equilibrium constants between selenium and magnetite have been obtained using the FITEQL program. For the case of Se(IV), the best fitting has been obtained using two inner-sphere complexes, FeOHSeO32- and FeHSeO3, while for Se(VI), the best fitting has been obtained considering only an outer-sphere complex, FeOH2+-SeO42-. The surface complexation reactions derived in this work are in agreement with those stated by other authors for sorption of Se(IV) and Se(VI) on hydrous iron oxides. (c) 2005 Elsevier B.V. All rights reserved.