Abstract Thermal analysis of clay and other substances at atmospheric pressure has developed into a major analytical method, beginning with the pioneering studies of Le Chatelier (1887) on kaolinite, more than a century ago. Thermal analytical methods differ from chemical or structural methods in that they rely on a phenomenological approach by investigating the response of material with respect to a change in temperature. If a material is investigated at constant pressure, but at different temperatures, the results represent observations using one independent variable (i.e., temperature). If successive experiments are carried out at different pressures as well, a two-dimensional pressure-temperature grid can be created, from which important information may be derived. The advantage of using pressure as an additional variable has not gone un-noticed, and different types of apparatus that allow high-pressure studies to be made have been developed (Wendlandt, 1986). The paucity of thermal analytical research at elevated pressures, however, suggests that these earlier methods are cumbersome. Recently, a relatively convenient high-pressure (≥ 10 kbar) differential thermal analysis method was developed in the authors’ laboratory (Koster van Groos, 1979). Because the apparatus is expensive to build and maintain, a simplified computercontrolled version is being developed for routine studies at moderate pressures < 2 kbar), which should facilitate wider usage of pressure in thermal analyses. Rather than review the literature extensively or describe the technique in detail, the purpose of this chapter is to illustrate that high-pressure differential thermal analysis (HP-DTA) is an important tool in solving geologic problems. One