Abstract: There is disclosed a class of inhibitors for mono- and diammonium phosphates which are water soluble and effective at small concentrations rendering the phosphates acceptable in most utilities without separation of the inhibitor. The class of inhibitors are water soluble alkali metal phosphate salts selected from sodium tripolyphosphate, sodium potassium tripolyphosphate, sodium acid pyrophosphate, sodium pyrophosphate, sodium hexametaphosphate, trisodium phosphate, disodium phosphate and monosodium phosphate. Effective amounts of these salts range from about 0.025 percent to about 0.3 percent by weight in the ammonium phosphates. It is particularly advantageous that these salts are useful in both types of ammonium phosphates.