Some of the porous sheet materials used in fuel cells and batteries hardly permit gas flow through the thickness of the sheet, although flow parallel to the sheet is appreciable. Determination of the porosity of such materials is not possible by the available techniques. A novel technique based on flow porometry is reported. This technique can measure the pore structure of such porous sheets. A composite porous sheet material containing one of the electrodes and the separator in two layers was investigated. The largest pore diameter, the mean flow pore diameter and the pore size distributions were measured. The pore structures of both layers were identified.