[en] Selenium-79 (half-life: 10⁶ years) is identified as one of the radionuclides that dominate radiological hazard in long-term performance assessment of geologic disposal for high-level radioactive wastes. We have performed dissolution experiments of metallic selenium under anoxic conditions (O₂ < 1 ppm) to acquire thermodynamic data of selenium. The measured concentrations of selenium were higher than the values calculated from a thermodynamic dataset. The equilibrium constant of the dissolution reaction of metallic selenium was obtained based on the expected dissolution reactions of metallic selenium and involving selenium species. (author)