Thermal evolution and sintering behavior of LaPO 4 ·nH 2 O, prepared by two different chemical synthesis routes, are presented in this work as a comparative study. The presence of polytrioxophosphate (La(PO 3 ) 3 ) in the as-synthesized powders, as minor second phase, and its consequences on thermal and sintering behavior are also discussed.At temperatures ~1000°C, the samples suffer a weight loss that could be caused by the volatilization of P 4 O 10 during the incongruent melting of the polytrioxophosphate. Furthermore, the derivative curves of the linear shrinkage show peaks at temperatures higher than 1000°C that can be also related to that process. HREM–SAED–XEDS showed an amorphous phase in samples calcined at 800°C/12h that surrounds the boundaries of La(PO 3 ) 3 crystals and according to XEDS analyses corresponds to LaPO 4 . In addition, a glassy phase is also detected at some grain boundaries in sintered samples after polishing and thermal etching.