Bentonites are candidate materials for high-level radioactive waste (HLRW) repositories and, therefore, are investigated with respect to long-term stability. In order to identify possible bentonite alteration processes, long-term in situ tests are conducted in rock laboratories. The prototype repository in situ experiment (PR) is one of the best examples of this kind of test due to the size of the installation as well as the duration. In the present study, chemical and mineralogical alteration processes of the bentonite MX 80 after an 8 y heating period were investigated. The water content of all samples increased following inflowing Na-Ca-Cl-type granitic groundwater causing cation exchange in the bentonite buffer materials. Exchangeable magnesium was desorbed in the buffer and MgO concentration increased at the bentonite–Cu canister interface; the Mg sink could not be detected, however. CaO also accumulated at this interface mainly as Ca carbonate and Ca sulfate. Cu corrosion products were identified at the bentonite–canister interface by chemical analysis, scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM-EDX), and differential thermal analysis. Up to 0.5 mm into the bentonites Cu could be detected by SEM-EDX. No cristobalite dissolution was observed in contrast to other in situ tests in which iron heaters were used. The corrosion products and the lubricant which was added during manufacturing of the bentonite blocks were mixed with the bentonite at the bentonite–canister interface. A quantitative measure of that mixture was the decrease in the cation exchange capacity (CEC). The CEC also reduced in all other samples, however, compared to the CECs of the reference samples, particularly in the warmer deposition hole 5 compared to the colder deposition hole 6. Overall, the PR in situ experiment proved that cation exchange reactions occurred in full-scale bentonite buffer experiments in all bentonite blocks but structural degradation of smectite could not be identified.