Spontaneous crystallization of gallium orthophosphate from 7.5 to 12 M orthophosphoric acid solutions has been studied at increasing temperature in the range of 150-260 degrees C. It was shown that the GaPO4 solubility in H3PO4 and H2SO4 solvents is retrograde in this temperature interval. The solubility hears (Delta H) of gallium orthophosphate were also measured.