The title heterolytic cleavage process of the NaCl microcrystal was studied by through ab initio calculations. First, a Na-Cl diatomic molecule was subjected to H2O association. However, the heterolysis of Na+ and Cl- was not achieved even after adding eight water molecules. Second, two cluster models, (NaCl)(m)(H2O)(n) (m = 4 and 6), were examined. At n = 6, an incipient dissociated model, where a pair of corner sodium and chloride ions leaves the NaCl cluster, was successfuly obtained. When two lone-pair orbitals of a water molecule are directed toward the two sodium ions, the acidity of the hydrogen atoms of the H2O molecule is enhanced, leading to a strong hydrogen bond with a chloride ion. The hydrogen-bond directionality compels the ion to leave the corner site. Thus, the heterolytic dissociation is initiated by the cooperative dissociation of two ions with the aid of polarized water molecules. The (NaCl)(4)(H2O)(9) model can describe a complete Na+ and Cl- dissociation.