A composite surface comprising evenly distributed silicon oxide islands on a gold substrate is described. The composite surface is prepared by evaporation of a thin (50 nm) gold layer on oxide-free (H-passivated) silicon, followed by thermal diffusion of Si through the Au layer, gradually forming islands Of SiO2 on the Au surface. The rate of Si diffusion through the Au, and hence, the rate Of SiO2 island formation, is controlled by the annealing temperature, the Si crystallographic face, the Au film thickness, and the contacting atmosphere. The Au-SiO2 composite surfaces can be used in applications requiring substrates patterned on a mesoscopic scale, while exposing two chemically dissimilar phases. One such application is shown here, namely, the formation of thiol-silane monolayers, for which the distribution of the different molecules in the resultant monolayer is determined by the substrate composition. The XPS controlled surface charging (CSC) method is used to establish a site-selective adsorption. The SiO2 islands are found to be rather labile, shifting and aggregating upon self-assembly of alkanethiol molecules on the Au exposed areas. Pretreatment of the islands with a long-chain silane stabilizes the morphology.