The gas diffusion electrodes made of electroconductive porous PTFE (polytetrafluoroethylene) sheets supporting Pt catalysts were used as the oxygen reduction cathodes for chlor-alkali electrolysis. It was clarified that the application of this cathode to the chlor-alkali electrolysis system lowered the cell voltage by about 0.8 V at 20 A/dm(2) and 90 degrees C as compared with the conventional hydrogen evolution cathode system. It was also found that the activity of the cathode was deteriorated by the decrease in the amount of Pt catalyst. This decrease is believed to be due to the oxidative corrosion of carbon carriers. The durability of the cathode comprised of the carbon carriers, which were resistant against oxidative corrosion, was more than 400 days.