Infrared (IR) spectroscopy, differential thermoanalysis (DTA), thermogravimetry (TG) and quadrupole mass spectrometry (QMS) have been used in combination to study the behaviour of GaPO4 crystals upon heating. During heating a continuous weight loss takes place nearly imperceptibly, although up to about 1000°C no TG, DTA or QMS signal can be observed. Then, however, sharp TG, DTA and QMS signals are observed, indicating the emanation of a certain amount of water. The weight loss coincides exactly with the transition from the low-quartz analogous into the high-cristobalite analogous structure. A linear relationship between the amount of water released at this transition temperature and the room temperature IR absorption coefficient is found. Some conclusions about the interplay between phase transition and weight loss will be considered.