The chemical interaction between ammonium polyphosphate (APP) and manganese dioxide (MnO2) was studied by thermogravimetry with linear heating. The products of interaction were collected at successive steps of weight loss and analysed by X-ray diffraction (on crystalline products) or by chemical analysis (on amorphous products). It was found that MnO, starts to interact with APP at the beginning of the thermal decomposition of APP at 270-280 degrees C. At the onset of weight loss of the APP/MnO2 mixtures, manganese(VI) is reduced to manganese(III) and (II) producing MnNH4P2O7 and Mn(NH4)(2)(PO3)(4) respectively. Ammonia, which is partially reduced to nitrogen, is evolved from PP. The formation of MnNH4P3O9 and then Mn2P4O12 is observed on further heating of the mixtures to 560-570 degrees C. The content and ratio of the products of reaction depend on the temperature and the ratio of components in the initial APP/MnO2 mixtures. Assignment of the X-ray powder diffraction peaks and an estimation of the lattice parameters have been made on orthorhombic MnNH4P3O9 (n = 7.357(3), b = 12.186(8), c = 9.390(5)Angstrom, space group P-mcn).