Sodium dihydrogenorthophosphate was heated at a temperature between 250 and 550 °C in dry air with humidity less than 1% and in moist air with humidity of 100%. Regardless of humidity, the orthophosphate gave the following products; Na₂H₂P₂O₇ at 250 °C, Na₂H₂P₂O₇ and NaPO₃II at 300 and 330 °C, NaPO₃I and NaPO₃II at 370 and 400 °C, and NaPO₃I at 450 to 550 °C. When NaH₂PO₄ was heated at 250°C for more than 5 h in the dry air and for more than 1 h in the moist air, NaPO₃I was also formed. The formation of NaPO₃I in the moist air was faster than in the dry air. The Na₂H₂P₂O₇ made above was reheated at 300 °C in the dry and moist airs. The product contained NaPO₃I and NaPO₃II. The content of NaPO₃I was larger than that of NaPO₃II regardless of humidity. NaPO₃l was not produced at all in the case of one-step isothermal heating of NaH₂PO₄ at 300 °C.