Fabrication of composite membranes from a highly porous substrate and a thin polymeric permselective layer allows achieving both high flux and selectivity in a single structure. In practice, formation of such composites is difficult due to the presence of large pores on the surface of highly permeable substrates. A novel approach to the formation of composite membranes was examined in this work to avoid this difficulty with large, hard-to-coat pores. Anopore microporous aluminum oxide membranes with 2000 Angstrom pores were used as model systems and were treated with 100 Angstrom spherical colloidal silica particles to reduce their pore diameter while maintaining low flow resistance through them. The silica-treated ceramics were then used as substrates for formation of thin defet-free polymer-ceramic composite membranes by a solution coating method. The treatment protocols, characterization of the treated substrates, and gas permeation results for the composites formed are discussed.