NOVELTY - A gas diffusion backing has a porous layer (1) with a microporous layer (5) in electrically conductive contact. The layer (1) consists of porous carbon paper or fabric having 50 vol% carbon fibers (2). The carbon fibers are at least partially coated by fluorinated polymer (IA). The layer (5) consists of fluorinated polymer (IIA) having carbon particles intermixed.    USE - For a membrane electrode assembly used in fuel cell (both claimed) to produce electricity.    ADVANTAGE - The gas diffusion packing has a thin microporous layer such that the mass transport of water is not impeded in the fuel cell. The water in the fuel cell is readily allowed to wick to the microporous layer and is removed from the catalyst surface, where excess quantities of water will reduce the efficiency of the fuel cell.    DETAILED DESCRIPTION - A gas diffusion backing comprises a porous layer (1) with a microporous layer (5) in electrically conductive contact. The layer (1) consists of porous carbonaceous paper or fabric comprising 50 vol% carbon fibers. The carbon fibers are at least partially coated by fluorinated polymer (IA). The layer (5) consists of fluorinated polymer (IIA) having carbon particles intermixed. The polymers (IA,IIA) are the same or different, each being a melt processable polymer selected from amorphous polymers having glass transition temperature of less than 250degreesC, and/or crystalline or semi-crystalline polymers having melting point of less than 315degreesC.    INDEPENDENT CLAIMS are included for the following:    (1) process for forming a gas diffusion backing comprising contacting a porous carbonaceous paper or fabric comprising carbon fibers with fluorinated polymer (IA) to impregnate the polymer into the paper or fabric and at least partially coat the fibers, to form a porous layer (1) containing 50 vol% or more of carbon fibers. A layer (5) comprising fluorinated polymer (IIA), carbon particles and components (P,Q) is applied to the layer (1). The component (Q) has boiling point greater than component (P). The layers (1,5) are dried to remove a low boiling solvent. The layers (1,5) are individually heated after the layers are brought into contact to form a gas diffusion backing having a microporous layer (5) in electrical contact with the first layer (1);    (2) membrane electrode assembly comprising a solid polymer electrolyte membrane, electrode(s) on surface(s) of the membrane and the inventive gas diffusion backing(s) adjacent to electrode(s); and    (3) fuel cell comprising the membrane electrode assembly.    DESCRIPTION OF DRAWING(S) - The figure shows a cross-sectional view of a gas diffusion backing.    porous layer (1)    carbon fibers (2)    fibrils (3)    microporous layer (5)