The adsorption of octanoic acid (Octac) and octylamine (Octam) onto silica from isooctane solutions has been studied, both from individual solutions of Octac and Octam and from mixtures of the two in isooctane. The results show that Octarn has a slightly higher affinity than Octac for the silica surface. From mixtures of Octam and Octac, the adsorption behavior at excess Octam is found to be different from that at excess Octac. In the first case the results indicate some kind of multilayer formation, while in the latter case it appears as though the two adsorbates compete for the same adsorption sites on the silica surface. When the equilibrium concentrations of the two components approach the equimolar, however, the adsorption behavior is similar for the two cases.