Cation exchange competition (CEC) is driven by water uptake during saturation of bentonite barriers surrounding canisters releasing heat from radioactive waste. CEC differences may be used to follow smectite degradation. The unanswered question is whether processes can be understood in more detail by studying a full set of 30 bentonite blocks of the Alternative Buffer Material (ABM) test series (ABM-5) after reaction in an underground laboratory operated in crystalline rock at temperatures of ~250°C, the highest reported temperature so far. In contrast to expectations, only a minor CEC decrease of, on average, 1.8 meq 100 g–1 was detected, although processes depending on high temperature were expected to alter the swelling properties of smectites that can be followed analytically by reducing bentonite CEC values. A critical role of initial water saturation and initially ~25% Na+/CEC on exchangers was identified by comparison with the first ABM-1 package where CEC decreased by on, average, 5.5 meq 100 g–1. ABM-1 was heated from the start whereas the packages ABM-2 and ABM-5 in this study were heated after water saturation. Exchangeable cations (EC) were distributed within the whole barrier in ABM-5 with (1) more pronounced horizontal EC gradients and (2) the absence of an exchangeable Na+ decrease. In all tests, a cation equilibration with the Äspö groundwater averaged over the whole packages of many different buffer materials was observed, showing, overall, a significant range in final composition after retrieval: Na+ (27–46%/CEC), Mg2+ (7–15%/CEC), and Ca2+ (45–100%/CEC). The groundwater for saturation, however, was locally variable in composition. Although excluded from the smectite interlayer (below or equal to 2 water layers), Cl– entered the barrier from groundwater, increased significantly in nearly all ABM-5 bentonite blocks, and was found to be mobile also in the less heated ABM-1 and ABM-2 test packages.