Saponitic bentonite mined in the Magan deposit (Toledo, Spain), has been classified as a suitable clay barrier in the storage of high-level radioactive waste. Several hydrothermal alteration assays have been carried out in Teflon reactors at 45, 60, 90, 120, 175 and 200°C for periods of up to 1 y. The mineral components of bentonite are stable below 175°C. At and above this temperature, the accessory sepiolite transforms into a monomineral phase of saponitic composition. The texture of the clay also changes. A rise in temperature above 120°C causes a decrease in the proportion of the <2 μm size-fraction, a reduction of BET and total surface areas and an increase in the relative volume of micropores (<20 Å ). This process has been interpreted as the formation of granular aggregates that preserve a micropore network. This new arrangement of the aggregates produces a significant reduction in the free swelling volume.