Crystals of millimetric dimensions of layered -Zr(HPO 4 ) 2 • H 2 O start to lose hydration water at temperatures higher than 270°C when heated at rates of 2-50°C/min. This allowed the observation, at about 143 and 257°C, of two new reversible phase transitions of monohydrate zirconium phosphate. The connected enthalpic effects were 3.8 ؎ 0.2 and 2.7 ؎ 0.1 kJ/mol, respectively. Measurements of the thickness of the crystal, along the perpendicular to the layers, were also performed as a function of the temperature. At temperatures very close to those of the two-phase transitions, the thickness of the crystal decreased by 0.69% and by a further 0.44% of its initial value. A thermal behavior very similar to that found for large crystals was observed when microcrystalline -Zr(HPO 4 ) 2 • H 2 O (average crystal size 1-10 m) was analyzed at very high heating rates (i.e., 50°C/min). X-ray powder diffraction analysis revealed that the phase transitions involve a discontinuous decrease of the interlayer distance from 7.56 to 7.50 A s at 134°C, and to 7.43 A s at 257°C, in agreement with dilatometric data. Finally, some structural aspects involving the new phases and the possible relations with those already reported are discussed.