The equilibrium between water and a cerium dioxide powder was studied by means of thermogravimetry between 373 and 573 K. The isotherms can be interpreted by the additive contribution of water adsorbed on two distinct sites which differ in their standard enthalpy of fixation and their relative population. The results agree with the model water is fixed on the ceria surface through direct bonds to cerium atoms and by hydrogen bonding to hydroxyl groups located in the micropores. The latter sites disappear following calcination at high temperature. This is confirmed by transmission electron microscopy photographs and nitrogen adsorption measurements