Abstract: X-ray diffraction, infrared, and cation-exchange capacity measurements of the reaction products of montmorillonites with YbCl3·6H2O show that at 1 atm irreversible sorption of Yb3+ increases with increasing temperature in the range 20° to 280°C, whereas at 110atm it decreases with increasing temperature. Above 100°C, less irreversible sorption occurs at 110 atm than at 1 atm. The decreased sorption at high pressure is attributed to reduced cation hydrolytic fixation and to rapid expulsion of interlayer Yb3+ by interlayer water at higher temperatures, with a concomitant decrease in Yb3+ migration to octahedral sites. At 110 atm, 160° and 200°C treatments cause changes in infrared absorption bands (884 cm−1, 848 cm−1) suggesting that sorbed Yb3+ is charge compensated by the deprotonation of Fe3+- and Mg2+-hydroxyl groups. At 290°C deprotonation is restricted to Fe3+-hydroxyl groups.