Chemical preparation, crystallographic characterization, thermal behavior and IR studies are given for two new cyclotriphosphates SrTlP3O9.3H(2)O and SrTlP3O9.SrTlP3O9.3H(2)O is orthorhombic, space group Pnma, with the following unit-cell dimensions: a = 9.147(7) Angstrom, b = 8.180(7) Angstrom, c = 15.458(2) Angstrom and Z = 4. The total dehydration of SrTlP3O9.3H(2)O leads between 300 and 650degreesC to its anhydrous form SrTlP3O9.SrTlP3O9 is monoclinic, space group P2(1)/m or P2(1), with the following unit-cell dimensions: a = 14.544(2) Angstrom, b = 8.639(1) Angstrom, c = 7.727 (1), Angstrom, beta = 102.05 (1degrees) and Z = 4. The thermal behavior has been investigated and interpreted in agreement with IR absorption spectrometry and X-ray diffraction experiments. We calculated the 30 normal frequencies of the P3O9 ring with Cs symmetry and proposed the interpretation of the vibrational spectrum of SrTlP3O9.3H(2)O. The vibrations were assigned and precised to each frequency for different atoms of the ring on the basis of the results of the theoretical isotopic substitutions and in the light of the crystalline structure of the isotypic compounds, (SrMP3O9)-P-1.3H(2)O (M-1 = Rb+, K+ and NH4+), of SrTlP3O9.3H(2)O. (C) 2002 Elsevier Science Ltd. All rights reserved.