KH(2)PO(4), K(2)HPO(4) and their mixts. were thermally converted to K polyphosphates at 225-450 degrees C under conditions of differential thermogravimetry and differential scanning calorymetry. Formation of K(3)H(3)(PO(4))(2), K(5)P(3)O(10) and their dihydrates was evidenced. As by-products, K(10)P(6)O(20), K(3)PO(4) and K(4)P(2)O(7) were formed.