Crystals of the trihydrated gadolinium diphosphate HGdP2O7. 3H(2)O were obtained from a mixture of diphosphoric acid and gadolinium nitrate solutions. This salt was characterized by X-ray powder diffraction, IR spectroscopy and thermal analysis. IR spectroscopic study reveals the existence of the characteristic band of the P2O7 group, the v(as)(POP) band, at 941 cm(-1) and the absence of the v(s)(POP) one. The thermal decomposition of HGdP2O7. 3H(2)O, which was followed by thermogravimetry and differential scanning calorimetry, shows that the elimination of the crystallization water takes place in three stages between 71 and 500 degreesC. The overall DeltaH of the first two dehydration were found to be 41.06 and 25.88 kJ/mol. New well-crystallized hydrates HGdP2O7.1.5H(2)O and HGdP2O7.0.5H(2)O were obtained through heating HGdP2O7. 3H(2)O, respectively, at 145 and 280 degreesC. The anhydrous HGdP2O7 salt, which was obtained by calcination of HGdP2O7. 3H(2)O at 530 degreesC, decomposes at 700 degreesC with loss Of P2O5 to give the gadolinium diphosphate Gd-4(P2O7)(3).