The invention relates to a composite structure consisting of a relatively long filtration bar comprising, from the outside, an ultra-thin layer ( 26 ) that is selectively permeable to hydrogen and made from palladium or silver alloy. Said layer is disposed on a permeable, rigid, refractory substrate consisting of a more or less solid body ( 30 ) that is covered with an intermediary thin layer ( 28 ) having a relatively smooth surface. The body ( 30 ) and the intermediary layer ( 28 ) are made respectively by sintering with fine and ultra-fine Inconel grains. A rigid metallic axial structure ( 32 ) is embedded in the body ( 30 ). Veinlets ( 31 ), which are made in the body ( 30 ) through the destruction of thermo-destructible wires during sintering, increase the permeability of the substrate. The invention is particularly applicable to hydrogen-producing combustible gas processors.