A composite membrane composed of a three-dimensionally ordered macroporous (3DOM) silica matrix and a proton conducting polymer electrolyte was prepared. The proton conductivity of the composite membrane was successfully improved by introduction of sulfonic acid groups on the 3DOM silica surface. On decreasing pore size of the 3DOM silica matrix, a greater improvement of proton conductivity was observed. 1x10(-2) S cm(-1) (470 nm pore) or 2.6x10(-2) S cm(-1) (200 nm pore) improvement was attained for the composite membranes filled with 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) polymer. These values are two orders of magnitude larger than those expected from the amount of sulfonic acid groups introduced on the 3DOM silica surfaces. This result suggests a formation of new proton conducting paths by an interaction between sulfonic acid groups on the 3DOM silica surface and those in the AMPS polymer electrolyte. (c) 2007 The Electrochemical Society.