NOVELTY - Fabricating composite membrane structure by depositing continuous and non-porous proton-conducting inorganic oxide layer on organic proton-exchange membrane comprises: (a) combining metal and hydrogen peroxide into starting solution; (b) neutralizing hydrogen peroxide in starting solution; (c) adding wetting agent to neutralized solution; (d) disposing neutralized solution onto one face of organic proton-exchange membrane to generate coating; and (e) curing the coating to form the inorganic oxide layer, where the inorganic oxide layer comprises at least one proton-conducting metal oxide.    USE - The method is useful for fabricating composite membrane structure for fuel cell.    ADVANTAGE - The method: reduces fuel cell crossover, thus improving fuel cell efficiency and power output, and operating direct methanol fuel cells at high temperature; utilizes inorganic layer that can be varied without changing the proton exchange membrane to provide stability and compatability with a desired fuel; and avoids the use of highly dilute fuels in water.    DETAILED DESCRIPTION - Fabricating a composite membrane structure for a fuel cell by depositing substantially continuous and non-porous proton-conducting inorganic oxide layer from a liquid solution on an organic proton-exchange membrane comprising a proton-conducting polymer, comprises: (a) combining a metal and hydrogen peroxide into a starting solution, optionally with removal of heat arising from formation of the metal oxide in the starting solution; (b) substantially neutralizing the hydrogen peroxide in the starting solution to generate a neutralized solution; (c) adding a wetting agent to the neutralized solution; (d) disposing at least some of the neutralized solution onto one face of the organic proton-exchange membrane to generate a coating; and (e) curing the coating at a curing temperature to form the inorganic oxide layer, where the inorganic oxide layer comprises at least one proton-conducting metal oxide.