Chemical behavior of selenium (Se), which was one of the important elements for performance assessment of geological disposal of high-level radioactive waste, was investigated under reducing and iron-containing conditions. A washing method for an iron diselenide (FeSe2(cr)) reagent with acidic and basic solutions was carried out for the purification of FeSe2(cr) reagent, which was considered to be a solubility limiting solid for Se under the geological disposal conditions. Furthermore, solubility of FeSe2(cr) was measured in alkaline solutions under reducing conditions, and thermodynamic data on equilibrium reactions between Se in solution and Se precipitate were obtained. It was found that the dependencies of solubility values on pH and redox potential (Eh: vs. standard hydrogen electrode) were best interpreted assuming that the solubility limiting solid was not FeSe2(cr) but Se(cr) and the aqueous species was SeO3
2- in the present experimental conditions. The equilibrium constant between Se(cr) and SeO3
2- at zero ionic strength was determined and compared with literature values. The chemical behavior of Se under geological disposal conditions was discussed.