An isochoric method is described by which hydrothermal measurements may be made up to 7500 psi at 25 degrees C. or 4000 psi at 400 degrees C. for 1) solubilities of minerals, 2) mineral synthesis, 3) P-V-T relations of gases and liquids, 4) conductivities of liquids, and 5) rates of reaction where traceable by changes in pressure or solubility. Solids, a gas, and a liquid are added in weighed amounts to the evacuated reaction vessel without contamination and then agitated by rocking while being heated. Leaching of the chrome-plated, stainless steel vessel by aqueous 4.1 m NaHS solutions to 200 degrees C. added only 10 ppm Ni and 15 ppm Cr as background. Solubilities are determined by analysis of samples of either liquid or gas phases which are taken by a double valve system in a 2 step process; small (5 ml) samples are first separated at pressure and temperature from a large reaction vessel (about 1100 ml capacity) by a nearly isothermal, isobaric process, and then are withdrawn into a glass container near standard conditions. More than 12 samples may be extracted at various temperatures and pressures from one charge of the vessel with reproducibility of duplicates within the analytical error of + or -2%.