The vapor adsorption of different n-alkanes, cyclohexane, toluene, and water on bare and methylated pyrogenic silica (Aerosil OX-50) has been studied gravimetrically. Linear adsorption isotherms of the n-alkanes and of cyclohexane on both substrates are found up to high relative vapor pressures; the same holds true for toluene on methylated silica. The linearity of the isotherms indicates relatively weak lateral interactions. On bare silica, the adsorption, expressed in mol/m(2), of the n-alkanes studied (C7-C9) is independent of chain length. The adsorption strongly increases after the coverage, corresponding to a monolayer of alkanes oriented perpendicular to the surface has been reached. Methylation of the silica is found to decrease the adsorption of all the adsorptives studied. On methylated silica the octane isotherm lies below the coverage of a monolayer parallel to the surface until just before saturation. From the adsorption data, surface pressure isotherms are constructed, and the work of adhesion is obtained. The work of adhesion reveals that the Lifshits-van der Waals part of the silica surface tension is reduced from 44 mJ/m(2) for bare (pyrogenic) silica to 30 mJ/m(2) for methylated silica. The adsorption data are also converted to disjoining pressure isotherms. At small film thicknesses, the disjoining pressure can be described by an exponential short-range interaction.