[en] The chemical evolution of a bentonite in contact with two different aqueous solutions from Swedish granititc massifs has been studied by geochemical modelling of mass transfers. The initial solutions are natural solutions from drillholes, one shallow diluted groundwater and one more saline solution from a deep aquifer in the granite. In order to model the behavior of bentonite in a repository for spent nuclear fuel (SKB project, Stockholm), the clay/solution ratio was taken very high (4 kg/kg or 10 moles of bentonite/kg H₂O). The solubility products of the reacting bentonite has been estimated from 25 to 200 degrees C using a solid solution model. With these datas the calculations showed that the bentonite does not remian stable with respect to the solutions along the heating curve (25 to 100 degrees C). The alteration of the bentonite may change a lot the chemistry of the solutions and produce a secondary clay (smectite) and calcite, zeolite (laumontite) and amorphous silica. The mass transfer is always less important for the most saline water. However, these mass transfers and chemical evolutions of the solutions corresponds to an overall limited degradation of the bentonite phase because of an interne self buffering effect due to a high clay/water ratio. In particular no evolution to an illite type clay has been detected with no evidence for a potassium uptake in the exchangeable sites in a closed clay + water system. (author)