An apparatus and method for determining the solubility of silica in superheated steam at high pressures are described. At 752 F (400 C), the solubility of quartz in steam increases from 1.0 ppm at 500 psi to 1548 ppm at 15,000 psi; at 932 F (500 C), the solubility increases from 4.3 ppm at 500 psi to 2596 ppm at 15,000 psi. The curves representing the solubility results at 752 F and 932 F cross at two pressures. Between these two pressures the solubility is greater at 752 F than at 932 F; below and above these pressures the solubility is greater at 932 F than at 752 F. This crossing is ascribed to the pressure-density relationships of water at the two pressures, which are affected by the proximity at 752 F of the critical point of water. When the solubility is plotted against the density of steam at each measured point, the curves do not cross, and the solubility at 932 F is greater than at 752 F. The solubilities of some other substances in superheated steam at high pressures are given also.