The chemical structure of the silica surface plays a key role in the adsorption and bonding of organic molecules from aqueous solution. This paper summarizes the known variables in surface structure, such as the state of hydration and dehydration, ionic charge, and the presence of aluminum as anionic aluminosilicate sites. These variables affect the degree to which organic molecules are bound to the surface through the three known forces, namely, ionic attraction, hydrogen bonding, and hydrophobic bonding. The latter probably has not been sufficiently widely appreciated in the case of pyrogenic silicas which have surfaces that are partly hydrophobic.