Several emitter materials, tantalum, tungsten and rhenium, and two emitter geometries, planar and excess area were evaluated in a flexible laboratory thermionic converter design. Results are reported from six converters that were tested for a total of 1070 hr. Experimental results from the excess area emitter converters did not exhibit the current enhancement observed in multicapillary and slotted emitter converters developed on a previous Air Force contract. While the electron currents in the passive mode at relatively low cesium pressure were in qualitative agreement with theory, the actual current densities obtained were not high enough to produce practical power densities at high output voltage. The method of emitter fabrication which used a niobium braze material appears to be a primary reason for the low performance.