The dissociative adsorption of water on defective NaCl(100) surface is investigated by the embedded cluster method at the UHF/MP2 level of approximation. The potential energy surfaces associated with OH bond breaking are calculated on step sites and on vacancies (V-s and F-s centers). The barriers of activation on a step and on a V-s center are very high and cannot account for the dissociation of water on defective NaCl surfaces experimentally observed at 240 K. On the contrary, the barrier of activation on F-s centers fulfills this condition. An optimal adsorption geometry is proposed where all the products of the reaction are trapped into the vacancy. This model is compared to the EEL and NEXAF spectroscopy results.