Hydrothermal experiments were performed at 100 and 200°C and at different clay:water ratios in order to investigate the transformation of smectitic layers during the alteration of a montmorillonitic starting material. This study focused on three phenomena: (1) the amount and localization of charge within the layer of the newly-formed dioctahedral smectite; (2) the stacking of low- and high-charge layers in the dioctahedral smectitic material; and (3) the neoformation of trioctahedral smectites. In all of the runs, the formation of beidellite from montmorillonite induced morphological changes in clay particles which suggests a reaction proceeding by a dissolution-crystallization mechanism. Illite layers were detected in K-saturated montmorillonite runs after the transformation of ~50% of the starting montmorillonite into beidellite (i.e. after 5 months of reaction with distilled water at 200°C). These illite layers were interstratified with both high-charge and low-charge dioctahedral smectites in a hypothetical three-component mixed-layer mineral.