Application of a gas- diffusion type oxygen cathode will contribute to energy saving in chlor- alkali electrolysis. For this purpose the development of gas-diffusion electrodes with high performance and durability is essential. We have investigated the performance for oxygen reduction and the mechanism of its on gas-diffusion electrodes with and without Ag catalyst in order to develop such oxygen cathodes with high performance and durability. It has been found that an electrode with no catalyst, that is, carbon support only in the reaction layer, shows electrochemical activity for oxygen reduction in 32 wt % NaOH at 80degreesC and 1 atm O-2, but loading of 2 mg cm(-2) Ag of particle size 300 nm, not only improves the activity by about 100 mV but promotes the four- electron reduction to produce OH-, while H2O2 is the predominant reaction intermediate in the absence of the Ag catalyst. The production of H2O2 has been demonstrated by conducting CV measurements to detect H2O2 in the anodic scan after a cathodic sweep up to 0.3 V vs RHE. It has been shown that the gas-diffusion type oxygen cathode with Ag catalyst has the high performance and durability necessary for chlor- alkali electrolysis.