Phase relations around the oxyapatite phase in the Nd2O3-SiO2 system have been investigated by means of a slow cooling floating zone (SCFZ) method and solid-state reactions. The results on SCFZ proved that the oxyapatite phase melts congruently at the composition of Nd2O3:SiO2=7:9, corresponding to Nd-9.33(SiO4)(6)O-2. The 2:3 phase (Nd-8(SiO4)(6)), which had been believed to have an apatite structure and to melt congruently, does not exist and consists of a mixture of Nd-9.33(SiO4)(6)O-2 and Nd2Si2O7 Phases. The solid-state reaction revealed that a solid solution region of the oxyapatite phase is extremely narrow or does not exist below 1650 degreesC. (C) 2002 Elsevier Science B.V, All rights reserved.