Adsorption of nine different gases and vapors on a silica gel was studied based on ab initio MO calculations. A single-type adsorption site constructed by one silanol group, and a paired-type adsorption site by two silanol groups were used as molecular models for adsorption sites on the silica gel. Adsorption structure and interaction energy were determined for the molecular models, and a decomposition of the energy was carried out. Adsorption equilibria of the nine gases and vapors on the silica gel were measured, and differential heats of adsorption were determined by applying the Clausius-Clapeyron equation to the measured isotherms. The interaction energy estimated by the MO calculation roughly corresponded to the measured heat of adsorption. The single-type adsorption site is favorable for adsorption of ammonia. The interaction energies of ethylene were almost the same on both adsorption sites. In the adsorption of alcohols, water, benzene and sulfur dioxide, the paired-type adsorption site gave a more stable adsorption structure than the single-type site.