The solubility of gallium orthophosphate (GaPO4) in 15 M phosphoric acid has been studied in the temperature range between 150degreesC and 250degreesC. The data were determined using an experimental set-up, which was especially designed for the purpose of dissolution as well as crystallization experiments. The measured values are discussed with respect to the question whether or not a considerable region of metastability exists. The temperature coefficient of the solubility has been found negative for temperatures up to about 250degreesC. (C) 2002 Elsevier Science B.V. All rights reserved.