Hexa-ammonium tetraphosphate dihydrate, (NH4)(6)P4O13. 2H(2)O (HATP), was prepared by the hydrolysis of sodium cycle-tetraphosphate with sodium hydroxide solution, followed by ion-exchange with ammonium. Thermal decomposition in static air was first carried out dynamically, at a heating rate of 5 K min(-1) as used in thermal analysis (thermogravimetry-differential thermal analysis), and also isothermally. To examine the effect of humidity on the thermal decomposition, HATP was heated isothermally in streams of dry and humid air. The products were characterized by X-ray diffraction analysis and high-performance liquid chromatography-flow injection analysis. At 100 degrees C, HATP was decomposed to mono- and triphosphates and to 2 mol diphosphate, and this was accelerated by humidity. Further degradation of the triphosphate to mono- and diphosphates took place slowly. The 2 mol diphosphate also decomposed slowly to 4 mel monophosphate. At temperatures above 150 degrees C, the form I of ammonium polyphosphate (I-APP) was produced. I-APP was further hydrolysed by humidity to shorter-chain phosphates, such as mono-, di- and triphosphates.