The chemically favorable nature of bentonite pore water is clarified by the PHREEQE geochemical simulation code. Bentonite is viewed as a candidate buffer materialfor a high-level-waste repository, and bentonite’s pore water chemistry is expected to result in a reduced Eh and weak alkaline pH region. Pyrite (Fe2S), initially contained in bentonite, alters to magnetite (Fe3O4), and this redox couple reaction controls the oxidation reduction potential. A mild alkaline pH condition is produced mainly by an ion exchange reaction between the sodium in bentonite and the protons in the solution. A geochemical simulation of the ion exchange reactions and the pyrite-magnetite alteration suggests that a favorable chemical condition would exist during the waste glass dissolution and indicates that the pH and the Eh values are -7.5 to —9.4 and —450 to -320 m V, respectively, when the granitic groundwater intrudes into the compacted bentonite in the repository.