Bentonite is a basic component of most concepts of multibarrier systems in underground radioactive waste repositories. It is important to determine the bentonite stability under the conditions close to the future real situation. The paper brings the detailed mineral and structural analyses of smectites from the bentonitic material exposed to the long term Mock-Up-CZ experiment. The compacted barrier blocks and residual filling contained 85 % of bentonite from the Rokle deposit, 10 % of quartz sand and 5 % of graphite. They were exposed to temperatures of up to 90 °C for almost 4 years. Quantitative mineral analyses, crystal size distributions, FTIR spectra, as well as cation exchange capacity and layer charge density show high mineral stability of the Rokle bentonite under the conditions of Mock-Up-CZ experiment. Small changes in the crystal sizes and slight change in the layer charge as a consequence of the experimental alteration could be linked to the hydration and the variation of the geochemical environment of the experiment.