The solubility of cerium (III) phosphate in the system CePО40,5H2O - solutions of ex-traction phosphoric acids (EPC) and in the system СеРО₄·0.5H₂O - model solutions of H3PO4 depending on temperature and concentration of orthophosphoric acid has been studied. The values of solubility index pS from the pH value of the solution were found.