The aim of this study is to examine the effect of various high pH solutions on smectites. The starting materials were a Na,Ca-smectite and two homoionic Na-and Ca-smectites. The experimental solutions were 0.01 m NaOH (pH 12), 1 m K 2 CO 3 and mixtures of 1 or 3 m KCl with 10 À 4 or 10 À 2 m KOH (pH 10 or 12). To enhance the reaction rate, all experimental investigations were carried out at 150 jC (F 5jC max.), during 2 months. The internal pressure was 5 bars (liquid-vapour equilibrium pressure) when experiments were conducted in warm seal autoclaves and fixed to 150 bars, using standard cold seal vessels. The liquid/clay mass ratio was fixed to 10/1. Integrated data obtained by a multitechnique analytical approach [XRD, electron microprobe, transmission electron microscopy (TEM), scanning electron microscope (SEM) on run products, and inductively coupled plasma atomic emission (ICP-AES) and mass spectrometry (ICP-MS) on experimental solutions] show a distinct behaviour of smectite as a function of experimental fluid composition. In NaOH solution, run products remain low-charge smectites. In KCl + KOH solutions, crystal chemical changes concern mostly the interlayer. A significant replacement of Na by K, and a partial substitution of Ca by K are observed. The tetrahedral Si content does not change, but the total interlayer charge in smectite increases, balancing the changes occurring in the octahedral occupancy (release of Al and Fe). Low-charge smectite transforms into high-charge smectite and byproducts are quartz and feldspars. In the presence of 1 m K 2 CO 3 , homoionic Na-and Ca-smectites are unstable and in part replaced by zeolites (merlinoite), feldspars and calcium silicate hydrates (CSH), the latter being dominated by a tobermorite-like phase. The main consequences of the cement or concrete like pore fluid-bentonite contact at elevated temperature induce dissolution of smectite (at various rates), cation exchange and the formation of byproducts (quartz, feldspars, zeolites and CHS). These mineralogical changes affect the expandability and surface area of minerals, and therefore may significantly change the porosity and fluid flow diffusion in clay barriers exposed to high pH fluids.