Abstract: A microscopic theory of the kaolinite-water system is presented, based upon the assumption that the clay-water interaction may be envisioned as partly hydration of the exchangeable cations and partly adsorption by the oxygen and hydroxyl surfaces. The theory treats exchangeable cation hydration quantum-mechanically as an ion-dipole phenomenon and considers water adsorption by the mineral surface as a problem in hydrogen bonding. A statistical mechanical model incorporating the quantum-theoretical results is then invoked to find the contribution of each component interaction to the initial portion of the adsorption isotherm for homoionic kaolinite. Good agreement between the theoretical calculations and available experimental data is achieved for water vapor adsorption by Li-, Na-, K-, and Mg-kaolinite, without the use of ad hoc empirical parameters. The concordance, in turn, is used to suggest that the basis for adsorption hysteresis in kaolinite-water vapor systems is the irreversible transition: mineral surface water → cation hydration water.