A study of the high temperature (up to 1200°C) degradation of aluminum metaphosphate Al(PO3)3 has been carried out to evaluate the stability of this compound used for many purposes. For the preparation of Al(PO3)3, an aluminum dihydrogen-phosphate Al(H2PO4)3 solution has been heated at 700°C. Thermal degradation of Al(PO3)3 has been followed through the measurement of weight loss on isothermal mode at 1000 and 1200°C and by thermogravimetric analysis coupled with mass spectroscopy (TGA-MS). Structural data have been obtained by 27Al, 31P and 1H MAS NMR spectroscopy. NMR analyses have shown that [A] and [B] allotropic forms of Al(PO3)3 are formed after the preparation procedure. In addition, NMR revealed that hydroxyls groups are also present in the sample, and they form P-OH groups. TGA-MS showed that they are decomposed at a temperature of 875°C, while the decomposition of Al(PO3)3 into AlPO4 and P2O5 begins at 1000°C.