The immobilization of selenate (SeO ) using metal oxides (CaO and MgO) and ferrous salt as the immobilization reagents were examined by the leaching test and solid-phase analysis via XRD, XAFS, TGA, and XPS. The results indicated that nearly all of SeO was reduced to SeO in the CaO-based reaction within 7 days. Then, the generated SeO was mainly sorbed onto the iron-based minerals (Fe O and FeOOH) through the formation of both bidentate mononuclear edge-sharing ( E) and monodentate mononuclear corner-sharing ( V) inner-sphere surface complexes, suggested by PHREEQC simulation and EXAFS analysis. Differently, less amount of SeO (approximately 45.50%) was reduced to SeO for the MgO-based reaction. However, if the curing time increases to a longer time (more than 7 days), the further reduction could occur because there are still Fe(II) species in the matrix. As for the associations of Se in the solid residue, most of the selenium (SeO and SeO ) was preferentially distributed onto the Mg(OH) through outer-sphere adsorption. Definitely, this research can provide a deep understanding of the immobilization of selenium using alkaline-earth metal oxide related materials and ferrous substances. 4 4 3 3 2 3 4 3 3 4 2 2– 2– 2– 2– 1 1 2– 2– 2– 2–