Forty-nanometer-thin free-standing membranes with 55-nm-wide pores have been prepared via Langmuir-Blodgett transfer. Hydrophobized silica colloids and polyisoprenes with quaternary ammonium headgroups were jointly spread onto a water surface to form a hybrid monolayer in which two-dimensional ordered domains of particles are embedded in a polymer monolayer matrix. Photochemical cross-linking of the polyisoprene, followed by transfer to gold grids with 100-mum wide openings yielded free-standing hybrid membranes. Removal of the colloids gives rise to a free-standing membrane with pores of uniform size. These porous membranes might find application as improved filtration media or as masks or molds for surface patterning and nanostructure fabrication.