We determine the bulk liquid phase volumes in octadecyl-bonded silica (C 18 silica) columns equilibrated with acetonitrile–water and methanol–water (0–19%(v/v)) binary mixed solvents by a liquid chromatographic method with inorganic ions used as probes. The solvent composition and the thickness of the interfacial liquid layer formed on the C 18 -bonded silica surface are then determined from the bulk liquid phase volume, the total liquid phase volume, the surface area of the C 18 silica packing material, and the retention volumes of the isotopically labeled eluent components for the columns. We used two C 18 silica packing materials having identical bonding structures but different pore sizes and surface areas. Our results show that various hydrophilic organic compounds as well as inorganic ions recognize the interfacial liquid layer as being different from the bulk phase. The behavior of the solute compounds exhibiting substantially weak retention in reversed-phase liquid chromatography or the so-called negative adsorption, such as urea, sugars, and inorganic ions, can rationally be interpreted with a partition between the bulk liquid phase and the interfacial solvation liquid layer. The structural properties of the solvent layer on the C 18 -bonded layer determined by liquid chromatography are consistent with the molecular dynamics simulation results that have been obtained by other researchers.