Structural changes proceeding in gallium orthophosphate crystals under thermal treatment have been studied by means of thermoanalytical methods and X-ray di4raction. The samples were characterized with respect to their water content by means of IR absorption. The occurrence of structural changes has been investigated as a function of thermal history, annealing temperature, annealing duration, and cooling rate. Freshly crystallized material shows always the low-quartz analogue modi5cation. During the 5rst heating a transformation into the high-cristobalite form proceeds at about 9703C. This structural change is connected with a weight loss and the emanation of vapor phase species, both indicating the release of water from the samples. Cooling down the high-cristobalite form can lead either to the metastable low-cristobalite or the thermodynamically stable low-quartz modi5cation. The cooling rate is the deciding factor for the occurrence of one or the other phase. The stability range of crystals originally in the low-cristobalite form has been studied by various annealing procedures. It is concluded that the low-quartz structure is thermodynamically stable and can be deliberately obtained between room temperature and about 9303C, while the high-cristobalite modi5cation is stable at higher temperatures. The low-cristobalite form, observed at room temperature nearly without any exception after a 5rst high-temperature treatment, is only metastable. Its occurrence can be suppressed by proper thermal processing.