The aim of this work was to develop a porous film structure for an electrode gas diffusion layer (GDL) used for proton exchange membrane fuel cells (PEMFCs). This film was made from a matrix composed of two immiscible polymers filled with a mixture of electrically conductive materials fabricated via a twin-screw extrusion process followed by selective extraction of one of the two polymers. The matrix consisted of low-viscosity polypropylene and polystyrene (PS) and the conductive additives were composed of high specific surface area carbon black and synthetic flake graphite. The conductive blends were first compounded in a corotating twin-screw extruder and subsequently extruded through a flexible film die to obtain a GDL film of around having high electronic conductivity. The PS phase was then extracted with tetrahydrofuran (THF) solvent and a film of controlled porosity was generated. The morphology of the GDL porous structure was then analyzed by scanning electron microscopy. GDL porosity characterization was done by both Brunauer–Emmett–Teller (BET) and mercury-intrusion porosimeter. The effects of PS concentration and extraction time with THF on GDL porosity were also studied. Pore-size distribution obtained by BET and mercury-intrusion porosimetry revealed that the GDL structure is composed by both mesopores and macropores. Mesopores represent more than 60% of the total pore volume inside the GDL film.