The importance of the part played by solubility in the transportation and deposition of ore sulfides can only be assessed by experiment because thermodynamic extrapolation of solubility to high temperatures and pressures is unreliable. In the method of solubility determination described, zinc sulfide, tagged with Zn 65 , was dissolved by water in a bomb, the sulfide having been sealed into a small porous capsule to prevent solid particles from circulating in the bomb chamber. The gamma-radiation intensity of the dissolved ZnS was measured with a scintillation detector placed near the bomb. The solubility was obtained by reference to a standardization curve. The contents of the bomb were at a pressure of about 4,500 p.s.i. during the experiment, and the effective temperature of solubility was 130 + or - 5 degrees C. The accuracy of the method has not been fully established, but the writer suggests improvements in procedure and apparatus that he believes will make possible the determination of useful data for a wide variety of solid-aqueous systems at almost any pressure and temperature assumed for the formation of hydrothermal deposits.