Microscopic features of adsorption phenomena of a few alcohols and aromatic compounds on the silica surface have been investigated by the ab initio molecular orbital (MO) method. The interaction energies between adsorbates and silanol groups, and the structures of the adsorbate-adsorbent systems (adsorption geometries) are studied. In order to discuss heterogeneity of the surface, we consider the paired-type adsorption site, which is constructed by two silanol groups. It is found that the paired-type adsorption site is preferable for the adsorption of alcohols. As for aromatic compounds, interactions between substituent groups and silanol groups contribute much to the adsorption.