Many concepts for deep underground storage of high level radioactive waste include the use of bentonite (sodium-montmorillonite) as a backfill material surrounding the waste. It is therefore necessary to model the chemical conditions in the system sodiummontmorillonite-granitic groundwater-granitic rock in order to be able to predict the speciation and solubility of actinides and fission products in this environment. In this study, the chemical interactions and the evolution of the chemical composition of the water in such a system have been modeled using the geochemical computer code PHREEQE. The parameters considered are the pH, Eh, and the chemical composition in aqueous solution. Mineral formation has been taken into account in some cases. The speciation and solubility of uranium in this system have also been calculated.