Several authors have reported lower temperatures for the aragonite-calcite phase transition for the skeleton of scleractinian corals, when compared with those observed in the mineral aragonite. This phenomenon has been attributed to the hydration of the coral aragonite. Using differential scanning calorimetry coupled with thermogravimetry measurements, we have observed the existence of double endothermic peaks probably connected with two separate phenomena : water desorption, followed by the aragonite-calcite transition. Determination of the enthalpy of the first endothermic peak allowed us to predict the existence of hydrogen-bonding interactions between the oxygen atoms of the carbonate planes and the water molecules.