Thermal decomposition characteristics of lanthanide phosphates LnPO(4).H2O (Ln: La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y) prepared by crystallisation from boiling phosphoric acid solution has been presented. On the basis of the effects observed on DTA and DTG curves, while heated up to 1000 degreesC, the phosphates can be divided into two,groups: (a) the first group includes hexagonal (La --> Tb), and orthorhombic Dy phosphates, (b) the second group consists of tetragonal (Ho --> Lu) and Y phosphates. The group (a) is characterised by two endotherms below 300 degreesC, associated with dehydration. and one exotherm above 700 degreesC, associated with transformation of the hexagonal/orthorhombic into monoclinic modification. The temperature of the polymorphic transformation is increasing with the increase of the atomic number of the lanthanide from <750degreesC (La, Ce) to >900degreesC (Tb, Dy). The group (b) dehydrates within a wide temperature range (100-600 degreesC) and maintains the tetragonal crystal structure even after heating at 950 degreesC. However, an increased crystallinity, contraction of the a- and c-axes, and difference in IR spectra suggests topotactical structural changes in (Ho --> Lu) and Y phosphates. (C) 2003 Elsevier Science B.V. All rights reserved.