Melamine polyphosphate (I) was prepd. by reaction of melamine with aq. H(3)PO(4), optional filling with montmorillonite, and following thermal treatment at 330 degrees C in presence of CO(NH(2))(2) and then studied for thermal stability, structure and capacity of fire retarding polypropylene. Addn. of the I fire retardant (25%) resulted in increasing the O(2) index and decreasing flammability of the polymer.