The solubility of aluminium orthophosphate in cryolite melts was determined. Part of the binary phase diagram of the system Na3AlF6-AlPO4 was investigated. The eutectic point was determined to be at 43.7 mass% (or 57.2 mol%) AlPO4 and (696 +/- 1) degrees C. It is suggested that in pure molten cryolite melts the orthophosphate ion dissociates partly into a metaphosphate ion and an oxide ion. (C) 2006 Elsevier B.V. All rights reserved.