Crystals of the trihydrated gadolinium diphosphate HGdP 2 O 7 .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 P 2 O 7 group, the ν a s (POP) band, at 941 cm - 1 and the absence of the ν s (POP) one. The thermal decomposition of HGdP 2 O 7 .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 o C. The overall ΔH of the first two dehydration were found to be 41.06 and 25.88 kJ/mol. New well-crystallized hydrates HGdP 2 O 7 .1.5H 2 O and HGdP 2 O 7 .0.5H 2 O were obtained through heating HGdP 2 O 7 .3H 2 O, respectively, at 145 and 280 o C. The anhydrous HGdP 2 O 7 salt, which was obtained by calcination of HGdP 2 O 7 .3H 2 O at 530 o C, decomposes at 700 o C with loss of P 2 O 5 to give the gadolinium diphosphate Gd 4 (P 2 O 7 ) 3 .