The thermal dehydration and transformations of the cyclohexaphosphates Nd2P6O18 · 12H2O and Li3Nd3(P6O18)2 · 26H2O are investigated in the range 20–1000°C using differential thermal analysis and high-temperature x-ray diffraction, in conjunction with chromatographic determination of the anion composition of reaction products. The results demonstrate that, during heating, the cyclohexaphosphates convert into the polyphosphates Nd(PO3)3 (monoclinic and orthorhombic forms) and LiNd(PO3)4 (monoclinic form). These processes are not accompanied by P2O5 removal. The temperature stability limits of the polyphosphates are determined, and the possible mechanisms of the cyclohexaphosphate–polyphosphate transformations are discussed.