[en] The melting temperatures of monazite- and xenotime-type RPO₄ (R = rare-earth elements) are about 2000 degrees C or higher in air. These properties may make the rare-earth orthophosphates attractive new materials for high-temperature applications. There have been few systematic studies on the preparation conditions and thermal changes of the rare-earth orthophosphates. This paper, discusses the mechanochemical changes of weinschenkite-type RPO₄ · 2H₂O (R = Dy, Y, Er, or Yb) at room temperature and the thermal reactions of the new rhabdophane-type YPO₄ · 0.8H₂O and ErPO₄ · 0.9H₂O at temperatures ranging from room temperature to 1800 degrees C in air