This program studied the effect of various emitter materials and surface conditions in a plane parallel cesium vapor thermionic converter. Four thermionic converters were fabricated and tested. The first had a niobium carbide emitter and molybdenum collector. The second converter had an oxygen-treated tantalum emitter and a molybdenum collector. Experience gained testing the first two converters showed the desirability of modifying the converter design. The last two converters were improved in several details, but primarily each had an insulated guard ring surrounding the collector and provisions were provided so that during operation the collector and guard ring potentials and temperatures could be kept identical. Special care was taken to procure the purest materials and to clean and outgas all the parts of these converters. One of the new design converters had a tungsten emitter and nickel collector. The other had a rhenium emitter and nickel collector. Both had 0.005-inch separation between the emitter and collector.