As a part of the safety assessment of the geological disposal of high-level radioactive waste, the effects of dry density and exchangeable cations on the diffusion process of Na + ions in compacted bentonite were studied from the viewpoint of the activation energy for diffusion. The apparent self-diffusion coefficients of Na + ions in compacted Na-montmorillonite and in a Na-and Ca-montmorillonite mixture were determined by one-dimensional, non-steady diffusion experiments at different temperatures and dry densities. A unique change in activation energy as a function of dry density was found for the Na + ions in compacted Na-montmorillonite. The activation energy suddenly decreased from 18.1 to 14.1 kJ mol ร 1 as the dry density increased from 0.9 to 1.0 Mg m ร 3 , whereas it increased to 24.7 kJ mol ร 1 as the dry density increased to 1.8 Mg m ร 3 . Examination of the effect of exchangeable cations on the activation energies determined that the activation energies were almost constant, approximately 25 kJ mol ร 1 , for the montmorillonite specimens at a dry density of 1.8 Mg m ร 3 . However, three different activation energy values were obtained at a dry density of 1.0 Mg m ร 3 . These findings cannot be explained by the conventional diffusion model (the pore water diffusion model), which suggests that the predominant diffusion process alternates among pore water diffusion, interlayer diffusion, and external surface diffusion.