A study has been made to explain a type of hide-out of sodium phosphate in steam boilers operating at about 1700 psi. This has shown that an insoluble form of a sodium phosphate is stable at temperatures above 620 F. This type of phosphate is scale-forming and is resistant to heat transfer. This sodium phosphate does not have the properties corresponding to those of the sodium phosphates normally encountered in steam-boiler water. Analyses by chemical means, x rays, and petrographic all indicate that this is a different form of sodium phosphate. Tests indicate that no insoluble potassium phosphate is formed and, if suitable potassium-to-sodium ratio is maintained, this insoluble salt will not form.