The sorption processes of metals and semimetals on siderite are being studied because of the potential adequacy of the mineral for removing contaminants from natural waters and because of its formation during the reduction of iron(III) (hydr)oxides under anaerobic conditions in the presence of organic matter or bacteria. This transformation could involve the release of contaminants previously sorbed on the iron(III) (hydr)oxides. In this work, the sorption of selenite on natural siderite (Bordes de Conflent, Lleida, Spain) is experimentally studied and the results obtained are compared to the previous results on selenite sorption on magnetite, hematite and goethite.Results indicate that the process of sorption was very fast; the equilibrium was reached in less than 1 hour, which indicates that sorption will not be kinetically controlled. The experimental data were fitted with a non-competitive Langmuir isotherm, which would indicate that the main process for the decrease of selenite in solution would be its sorption on siderite via monolayer coverage of the solid surface. The values of maximum sorption capacity of siderite obtained at pH 7,1 and 8,0 were 6,9 10-7 and 6,4 10-7 mol g-1, respectively and the same value of Langmuir constant was obtained for both pH values, 4,7 104 L mol-1.