The mixture of rare earth oxide (Nd2O3 or CeO2), phosphorus compound ((NH4)(2)HPO4 or H3PO4), and an additive (urea: CO(NH2)(2), biuret: NH(CONH2)(2), cyanuric acid: (CONH)(3), or ammonium hydrogencarbonate: NH4HCO3) was heated at 700degreesC for 3 h. And then thermal products were analyzed by X-ray diffraction and Fourier transform infrared spectroscopy. In the system of Nd2O3-(NH4)(2)HPO4, the addition of urea had a little influence on fort-nation of Monazite-type neodymium orthophosphate: NdPO4, polyphosphate: Nd(PO3)(3), and ultraphosphate: NdP5O14. In the system of Nd2O3-H3PO4, urea worked as dehydrating agent in a certain region of CO(NH2)(2)/P ratio to form much P-O-P bonding. Thermal products changed from Monazite-type NdPO4 to Nd(PO3)(3) and from Nd(PO3)(3) to NdP5O14. The promotion of dehydration reaction was observed only in the system added urea, not in the systems added biuret, cyanuric acid, and ammonium hydrogencarbonate. In cerium salts, Monazite-type Ce(+III)PO4, Ce(+IV)P2O7, Ce(+III)(PO3)(3), Ce(+IV)(PO3)(4), and Ce(+III)P5O14 were formed without the addition of urea. The addition of urea prevented the formation of tetra-valent cerium phosphates. It was considered that ammonia produced from the decomposition of urea made up a reducing atmosphere and that in this atmosphere Ce(+IV) cation was reduced to Ce(+III) cation. (C) 2003 Elsevier Science B.V. All rights reserved.