A possibility of forming simple and complex phosphates containing Mn(III) was shown by the methods of solid-phase synthesis, the interaction of Mn(II) metaphosphate in molten alkali metal nitrates and spontaneous crystallization from phosphate melts with Mn(II) oxide and an alkali metal nitrate. MMnP2O7 (M = Li, Na, and K) double diphosphates, as well as Na2Mn2O(PO4)2 and Na2MnP2O7 double phosphates were synthesized and their physicochemical properties were studied.