Bentonites are considered as an engineered barrier for a high-level radioactive waste (HLRW) repository. The Czech product Bentonite 75 is a potential candidate material to be used as a buffer and a backfill. The bentonite barrier is supposed to undergo alteration and transformation processes due to reaction with groundwater and heating originated from radionuclide decay. Detailed knowledge of the bentonite-water interaction is a crucial issue for the evaluation of the long-term stability of the bentonite barrier. This study presents the results of a laboratory experiment in which the repository conditions, hydration, and heating of the bentonite were simulated. Bentonite 75 reacted in a closed system with demineralised water and groundwater at 90 °C for 2 years. Bentonite material (solid phase) and aqueous extracts were analysed over time with respect to the mineralogical and chemical changes. The results indicate that montmorillonite alteration, particularly illitization, could be excluded in this system due to the low concentration of K+ in the solution. In addition, cation exchange capacity measurements did not indicate any transformation process of montmorillonite. Slight modifications of montmorillonite basal reflection profiles in PXRD patterns were attributed to cation exchange reactions. The most reactive minerals included siderite, Fe-dolomite/ankerite, and calcite/Mg-calcite. Dissolution/precipitation of these minerals controlled the composition of the aqueous phase that affected the distribution of the exchangeable cations in the montmorillonite interlayer. The most significant change was the dissolution of siderite that induced partial dissolution of calcite, which resulted in the replacement of Na+ and Mg2+ by Ca2+. The overall evolution of the water chemical composition could be predominantly explained by mineral phase equilibria. The initial water composition did not affect the geochemical processes, which took place in the bentonite-water system due to the strong buffering bentonite capacity.