Hydrogen oxidation has been studied on a carbon-supported platinum gas diffusion electrode in a phosphoric aad electrolyte in the presence of carbon monoxide and oxygen in the feed gas. The poisoning effect of carbon monoxide present in the feed gas was measured in the temperature range from 80 to 150 degrees C. It was found that throughout the temperature range, the potential loss due td the CO poisoning can be reduced to a great extent by the injection of small amounts of gaseous oxygen into the hydrogen gas containing carbon monoxide. By adding 5 volume percent (v/o) oxygen, an almost CO-free performance can be obtained for carbon monoxide concentrations up to 0.5 v/o CO at 130 degrees C, 0.2 v/o CO at 100 degrees C, and 0.1 v/o CO at 80 degrees C, respectively.