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Extensive drying of smectites can cause the interlayer space to break down (collapse). This can affect the properties of bentonites as geotechnical barriers of HLRW (highly radioactive waste) repositories. If and to what extent the collapse occurs depends strongly on the type of interlayer cation. In particular K is known to lead to ready dehydration, in contrast to Ca and Mg. In the present study, various bentonites and one illite/smectite clay were dried/heated at 90°C for 1.5 a and in a different experiment at 120°C for 4.5 a and investigated with respect to mineralogical and geochemical changes of the smectite. Smectite alteration after extensive drying was restricted to changes of the exchangeable cations. The CEC decreased by 9% (90°C test) and 14% (120°C test). A slight decrease of exchangeable Na+ was observed following the 90°C test. No significant further decrease was observed after the 120°C test. In contrast, the larger cation exchange capacity (CEC) decrease after the 120°C test could be explained by increased Ca/Mg fixation. A possible mechanism for the observations is presented.