The adsorption from binary liquid mixtures of p-xylene and n-heptane at 25° on characterized rutile surfaces containing various amounts of molecular water and hydroxyl groups has been studied. A dual nature surface is indicated, one part (about 50 %) being hydrophilic and showing preferential adsorption of xylene, while the other shows preference for heptane. Marked dependence of the adsorption on the presence of molecular water on the surface is demonstrated. Equilibrium constants and surface activity coefficients have been calculated from the adsorption data using Schay's treatment based on a monomolecular model. For the dry surface the activity coefficients deviate significantly from unity, but removal of the specific influence of the surface hydroxyl groups with adsorbed water leads to a surface phase which is approximately ideal over the whole range of composition. The data are analyzed in terms of the various interactions involved.