The solubility of ammonium phosphate is reviewed. Many ammonium phosphates can be described in terms of the ternary system:NH3–PO5–H2O. However, this system differs from systems like the sulfates and halates in that it has a marked tendency to form condensed oligophosphate ions. The literature survey covers the period up to 1988. ©1998 American Institute of Physics and American Chemical Society.