A method for manufacturing composite membranes includes providing a branched polyalkoxy siloxane, providing an organic proton conductor, mixing the branched polyalkoxy siloxane with the organic proton conductor; and forming a membrane from the composite component mixture. Using the method according to the present invention, it is possible to increase the proton conductivity and the mechanical stability of membranes and to reduce the swelling by water and aqueous solutions. The obtained composite membranes may be used in PEM fuel cells.