Abstract: The composition of the gas phase was identified for the first time up to a temperature of 2200 K by the Knudsen effusion mass spectrometric method, and the partial pressures of the vapor species during the evaporation of hydroxyapatite were determined. Ca10(PO4)6(OH)2 was evaporated from a Knudsen effusion cell made of tungsten. In the temperature range 1200–1300 K, Ca10(PO4)6(OH)2 undergoes dehydration, forming Ca3P2O8, which evaporates in the form of PO, atomic calcium, and oxygen when the temperature further increases to 1750–2200 K.