The LiPO3-Y(PO3)(3) system has been studied for the first time. Microdifferential thermal analysis (mu-DTA), infrared spectroscopy (IR) and X-ray diffraction were used to investigate the liquidus and solidus relations. The only new compound observed within this system is LiY(PO3)(4), melting incongruently at 1104 K. An eutectic appears at 4 +/- 1 mol% Y(PO3)(3) at 933 K. LiY(PO3)(4) Crystallizes in the monoclinic system C-2/c with a unit cell: a = 16.201(4) Angstrom, b = 7.013(2) Angstrom, c = 9.573(2) Angstrom, beta = 125.589(9)degrees, Z = 4 and V = 884.5 Angstrom(3), which is isostructural to LiNd(P03)4. The infrared absorption spectrum indicates that this salt is a chain polyphosphate. (C) 2003 Elsevier Science Ltd. All rights reserved.