Abstract In order to obtain information on the completeness of potassium dihydrogen triphosphate topochemical ammonization the authors have studied the i.r. and Raman spectra of K 3H 2P 3O 10·H 2O, products of its interaction with gaseous ammonia and ammonium—potassium triphosphate (K:NH 4 = 3:2) precipitated from aqueous solution. The assignments of the absorption bands (2000-400 cm −1; 293 and 100 K) have been made. The spectra of double salts obtained by different methods, in contrast to their X-ray diffraction patterns, have been found to be identical. This is indicative of the completeness and partial topotaxity of acidic salt topochemical ammonization. The ammonization process can be investigated through the changes in intensity of the bands pertaining to the vibrations of the -PO 3H end group: v s PO * 2, vPO(H) and δPO(H).