The effect of FC3+ and Al3+, usually present as impurity ions in wet-process phosphoric acid, on formation and polymorphic transformation of sodium tripolyphosphate (STPP = Na5P3O10) has been investigated using X-ray powder diffraction, IR spectroscopy, and scanning electron microscopy methods. The simultaneous appearance of both high- (Form-I) and low-(Form-II) temperature modifications at the beginning of STPP formation at the temperature range of 250-300 degrees C in the impurity-free as well as in Al- and Fe-containing samples was recorded. Form-I in the impurity-free STPP, containing both crystallographic forms, underwent low-temperature transformation into Form-II around 350 degrees C, while Form-II was stable up to 450 degrees C and transformed into high-temperature Form-I above 450 degrees C. Ignition of the impurity-containing STPP revealed that Al3+ and Fell stabilized Form-I so that the low-temperature transformation (Form-I -> Form-II) did not occur, while Form-II transformed into high-temperature Form-I at a lower temperature (below 450 degrees C than in the impurity-free STPP temperature.