The hydration of calcium oxide and its control by superficial recarbonisation have been investigated by suspending a single pellet into an atmosphere of controlled humidity or carbon dioxide content for hydration and recarbonisation respectively. The reaction rate was monitored using a computer aided gravimetric method. The computerised system allowed the simultaneous measurement of the temperature and weight of a reacting pellet. It has been shown that the hydration reaction takes place on a sharp definite interface between the product layer and the unreacted core. A theoretical model for the hydration rate has been derived on the basis of the unreacted core model by introducing the effect due to the sample swelling. The model was solved and compared to experimental results data. Quite reasonable values were obtained for the chemical reaction rate constant and the effective diffusion coefficient through the product shell. Results suggested that gas phase mass transfer, porediffusion, and chemical reaction were all contributing resistances in the model, that is, the process was under mixed control. On the other hand, the results obtained clearly indicate that the recarbonated film increases the hydration resistance of the lime core.