Thermionic cogeneration is developing as a new energy conservation option. The characteristics of thermionic systems using combustion of natural gas as heat source are studied. The - until now unused - high temperature potential of fuels can be applied in order to extract work using the high temperature thermionic conversion process. The high temperature of the emitter and the required large heat flux to it demand a high radiation temperature of the burner walls. As natural gas can only just reach these requirements the resulting system efficiency drops to an intolerable low level if the combustion air is not preheated and the heat from the flue gasses is not recuperated. An analytical study shows the effects of using the collector cooling air to preheat the combustion air and the influence of the heat exchanging capacities of the recuperator and the burner walls. It is shown that using the collector cooling air leads to a partial increase of the system performance and that using a recuperator increases the performance to the maximum attainable value. The geometry of the recuperator is discussed.