The mineralogical evolution of di- and trioctahedral smectites (i.e. montmorillonite and saponite) exposed to high-pH environments has been studied to determine the influence of compositional differences on clay dissolution and the formation of new phases. The present study helps to gauge the effects of highly alkaline solutions on the swelling capacity of smectitic clays and experimental results are extrapolated to predict the behavior of smectite-rich soils in various technical applications such as nuclear-waste storage and architectural conservation. The present study revealed extensive dissolution of montmorillonite in 5 M NaOH or 5 M KOH solutions and the neoformation of various zeolites, thereby reducing the clay’s swelling capacity significantly. Saponite, in contrast, experienced less pronounced changes, including transformation into a randomly interstratified saponite-chlorite and a Si-rich amorphous phase. These changes only provoked a partial reduction in swelling capacity. The results imply that under repository conditions (e.g. alkaline environment caused by hyperalkaline fluids released during concrete leaching), the slow and limited transformation of saponite into corrensite-type minerals would be beneficial for preserving the clay’s swelling capacity and, therefore, its effectiveness as a sealing material. Conversely, the loss of swelling capacity as a result of zeolite formation in montmorillonite observed in the present experiments would limit the clay’s effectiveness as a sealing material in waste repositories. In the case of earthen architecture conservation, alkaline consolidation treatments aimed at reducing the soils’ swelling capacity and, thereby, improving water resistance, would only be effective for treating earthen structures made of soils rich in dioctahedral smectites. Soils containing trioctahedral smectites, in contrast, are not likely to improve their water resistance because the swelling capacity will only be partially reduced.