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The mobile-phase volumes ([V.sub.m]) in reversed-phase liquid chromatography (RPLC) with alkyl-bonded silica, defined as the difference between the total volume of eluent in the column ([V.sub.0]) and the volume of the eluent solvent layer formed by solvation of the bonded phase ([V.sub.L]), are determined by the method derived from the eluent electrolyte effect on the retention of ionic analytes. The validity of the [V.sub.m] values obtained is evaluated by comparing them with the retention volumes of various organic compounds and inorganic ions, which have been suggested as unretained markers, and those obtained from a linear dependence of the logarithmic retention factor on the carbon numbers of homologous series. From the results obtained, it has been concluded that the solvated liquid phase on a column packing material should be assigned to a part of the stationary phase and the method developed for determination of the [V.sub.m] value based on the ion partition model gives the most reasonable value as the mobile-phase volume in RPLC. The volume and the solvent composition of the solvated liquid phase on [C.sub.1], [C.sub.8], and [C.sub.18] bonded silica are estimated, and the effects of organic modifiers and the physicochemical structures of the packing materials on these values are discussed.