Batch reactor experiments were performed at 35 degrees and 80 degrees C to determine the effect of high molar KOH solutions on the mineralogy and solution chemistry of two different smectites. The Ceca smectite is a nearly pure montmorillonite, and the Ibeco clay has a significant component of 35% beidellitic charge. Each reactor was charged with 1 g of <2 mu m fraction smectite and 80, 160, 240 ml of 0.1-4 M KOH solution. In these experiments, the changes in solution composition and mineralogy were monitored as a function of time. In the experiments, at all temperatures and under all chemical conditions, the smectite appears to change in diffracting domain size, which decreases with time. This probably reflects a change in crystal shape. The structures then become more illite rich and the diffracting domain continues to decrease.