This paper describes the synthesis of some thorium phosphate compounds with different Th/P ratio (1/2, 2/3 and 3/4) by a spray pyrolysis technique. The so-prepared rough compounds were annealed at different temperatures for 2 h and then analyzed by mainly X-ray diffraction on powder and infrared spectroscopy. Every rough compound is composed by very badly crystallized ThO2 phase polluted by carbon residue. An annealing treatment at 800 degreesC leads to the thorium diphosphate phase, alpha-ThP2O7 in every case. At 900 degreesC, such a phase is decomposed into a thorium phosphate diphosphate phase (Th-4(PO4)(4)P2O7, called TPD). However, a thorium excess in the initial mixture (Th/P = 3/4) leads also to observe the ThO2 phase. The TPD phase is stable up to 1200 degreesC and does not react with the ThO2 compound. Beyond 1200 degreesC, the TPD phase is slowly decomposed into a thorium phosphate compound which should be a thorium oxide phosphate; this compound does not contain any diphosphate species. (C) 2004 Elsevier Ltd. All rights reserved.