In this paper, the effects of oxygen on the performance of and the emitter material transport in a planar thermionic converter were investigated. The oxygen and cesium partial pressures in the interelectrode gap were varied parametrically, and their effects on the operational characteristics of the converter were quantified for both isothermal and isoflux emitter heating conditions. With isothermal heating, a significant increase in the electric power density in the presence of oxygen (by a factor of 3-10, depending on the emitter temperature and the initial composition of the collector surface) was predicted. The load voltages corresponding to the maximum output power were similar to 0.3-0.5 V lower than those corresponding to the optimum efficiency. With an isoflux emitter heating of 20 W/cm(2), the introduction of oxygen into the interelectrode gap of a converter with an initially pure tungsten collector increased the electric power density by only similar to 19%; however, no improvement occurred for-an initially oxidized tungsten collector.