Specially designed "hydrothermal solution" equipment was used to determine the solubility of anhydrite H 2 O from 100 degrees C. to 275 degrees C. and from 1 bar to 1000 bars. The solubility of anhydrite decreases with increasing temperature and increases with increasing pressure. The hydrothermal solution equipment consists of a deformable teflon sample cell held in a stainless steel pressure vessel and sealed in such a way as to prevent interchange of material between the sample cell and the steel bomb. Separate pressure lines leading to the sample cell and to the steel bomb allow liquid to be pumped into or taken out of each container independently. Liquid and solid phases may be allowed to come to equilibrium in the teflon cell at constant temperature and pressure. The experimental mixtures are stirred by means of a teflon-coated bar magnet turned by an externally applied pulsating magnetic field. An internally filtered liquid sample can be withdrawn without significant disturbance of the equilibrium temperature and pressure, by pumping the liquid into the steel vessel at the same rate that saturated solution is removed from the teflon cell. From the solubility data obtained the following geologic conclusions can be drawn. Initially saturate solutions of anhydrite in water, migrating toward the Earth's surface, would become undersaturated. The increase in solubility caused by decreased temperature more than compensates for the decrease in solubility caused by decreased pressure. Anhydrite would precipitate from saturated solutions in sediments as burial takes place and the temperature rises, or from ground waters moving downward to regions of higher temperature. Anhydrite would precipitate from saturated solutions moving from a region of high pressure to a region of low pressure, as would be the case in rocks near openings toward which a fluid pressure gradient exists.