Based on available experimental data on the interaction of sodium bentonite and groundwater, a model has been developed that represents a means of extrapolation from laboratory data to the conditions in compacted bentonite. The basic reactions between sodium bentonite and groundwater are described by an ion exchange model for sodium, potassium, magnesium, and calcium. The model also assumes equilibrium with calcite and quartz. The calculations are carried out for two types of granitic groundwater: the Swiss reference groundwater (ionic strength I = 0.24 M) and the standard Swedish groundwater (I = 0.0044 M). It is calculated that the pore water of compacted sodium bentonite will have a pH of 9.7 and a carbonate activity of 8 × 10−4 M if the dry bentonite is saturated with Swiss reference groundwater; it will have a pH near 10.2 and {} = 8 × 10−3 M for standard Swedish groundwater. The long-term situation, which is important for nuclear waste disposal, is modeled by the assumption that the near field of a radioactive waste repository behaves like a mixing tank. It is calculated that sodium bentonite will be slowly converted to calcium bentonite over a long period. The model is used to calculate short- and long-term maximum solubilities of thorium, uranium, neptunium, plutonium, americium, and technetium in the near-field pore water of a potential Swiss nuclear waste repository. The redox potential in the near field is assumed to be controlled by the corrosion products of the iron canister. Using a conservative chemical thermodynamic data base, the maximum solubility of thorium is calculated to be between 2 × 10−10 and 10−8 M, that of uranium between 3 × 10−11 and 3 × 10−8 M, that of neptunium between 10−9 and 10−5 M, that of plutonium between 3 × 10−10 and 4 × 10−5 M, that of americium between 2 × 10−7 and 5 × 10−5 M, and that of technetium will not exceed 10−9 M.