[en] The redox condition of near-field is expected to affect the performance of engineered barrier system. Especially, the oxygen initially existing in the pore space of compacted bentonites strongly affects the redox condition of the near-field. For assessing the influence of the oxygen, the transport parameters of it in the compacted bentonite and consumption process should be known. Therefore, following researches were conducted. In order to understand the diffusion of dissolved oxygen (DO) in compacted bentonite and to predict the effect of DO, the effective diffusion coefficients of DO in compacted sodium bentonite were measured by electrochemistry. As the results, the following relationship between the dry density of compacted sodium bentonite and the effective diffusion coefficient of DO in compacted sodium bentonite was derived: De=1.53±0.13x10^-9 exp(-2.15±0.24x10^-3ρ) where De is the effective diffusion coefficient (m² s^-1) of DO in compacted sodium bentonite and ρ is the dry density (kg m^-3) of compacted sodium bentonite. The oxygen concentration in the bentonite is expected to be controlled by oxidation of pyrite as impurity in the bentonite. In order to investigate the above idea, the rates of pyrite oxidation by DO in compacted sodium bentonite were estimated from the experimental data on pyrite-bentonite systems using the obtained effective diffusion coefficient of DO. The results show that the averages of the rate constants of pyrite oxidation by DO in the bentonite for dry densities of 0.8, 0.9, 1.0, 1.1 and 1.2x10³ kgm^-3 were 1.38±0.32x10^-8, 1.10±0.24x10^-8, 1.16±0.35x10^-8, 9.36±2.23x10^-9 and 7.48±1.92x10^-9 m s^-1, respectively. The relationship between the dry density (ρ) and the rate constant (k') was expressed as follows: k'=3.94±1.06x10^-8 exp(-1.33±0.28x10^-3ρ). Whereas the rate constant in a carbonate-buffered solution (pH=9.24) was 1.46±0.09x10^-9 m s^-1. During oxidation of pyrite, an oxidized coating forms on the pyrite surface depending on pH of solution. The oxidized coating may retard DO diffusion into the interior of pyrite. It is important to identify corrosion products on pyrite during oxidation. Therefore the laser Raman spectroscopy was performed on pyrite surface before and after experiments. Consequently, the main oxidized products were ferric hydroxide and hematite. (author)