Amorphous calcium phosphate (ACP) is one of the major components of biological mineralisation and plays a crucial role in several biomaterials. However, due to their instability, the thermochemical characteristics of ACP and of the derived metastable phases are still unknown. The thermal transformations affecting ACP, with a Ca/P atomic ratio close to 1.50, were studied in the range 373–1173 K. The samples were examined by X-ray diffraction, FTIR spectroscopy and BET measurements. The thermal behaviour was analysed by thermogravimetric and differential thermal analysis. The dissolution enthalpies of the products, in 9 wt% aqueous nitric acid solution, were determined using an isoperibolic calorimeter. The samples remained amorphous up to 773 K. The evolution of the dissolution enthalpy between 298 and 573 K was attributed to the irreversible loss of adsorbed water. The adsorption enthalpy at 298 K of liquid water on ACP was estimated at −12.3 kJ mol⁻¹. At 673 K, the variation of the dissolution enthalpy corresponded to the occurrence in the amorphous phase of a more stable ionic organisation. The nucleation of the crystalline phase was believed to begin at 773 K. At 873 K and above, the ACP crystallised mainly into metastable α-tricalcium phosphate (α-TCP). Though the enthalpy of dissolution of the formed α-TCP varied with temperature, it remained lower than that of the most stable tricalcium phosphate crystalline structure: β-TCP. This phase was formed at 1173 K. The thermal measurements confirm the high instability of ACP compared to other calcium phosphates, including apatites. These properties, plus the very high specific surface area, explain the very fast transformation of ACP into apatite in aqueous media and justify its use in new types of self-setting cements.