The performances of grooved emitter (GE), grooved collector (GC) and grooved electrodes (GEL) thermionic converters are investigated, and the results are compared with those of an identical converter with smooth electrodes (SEL), which is tested at the same conditions. These converters, with planar polycrystalline molybdenum electrodes and a 0.5 mm inter-electrode gap, are tested at emitter temperatures, T-E = 1473-1673 K, collector temperatures, T-C = 773-1023 K and Cs pressures, P-Cs = 10-500 Pa. The grooved electrodes have concentric macro-grooves 0.5 mm wide, 0.5 mm deep and 1.0 mm apart. Measured and calculated performance parameters include ignition voltage, V-IG, barrier index, V-B, electric power density, P-D, and conversion efficiency, eta, as functions of the cesium pressure and electrodes temperatures. V-IG of the GC converter is smaller than that of the SEL converter by up to similar to0.9 V for P-Cs = 20-100 Pa. VB for the SEL converter is always lowest, except at T-E = 1673 K and T-C = 1023 K, indicating that grooved electrode(s) are effective in reducing V-B only at high electrodes temperatures. The GE converter has the lowest P-D and eta, followed by the GEL converter, GC converter and, finally, the SEL converter, except for T-E = 1473 K and T-C = 773 K and for T-E = 1673 K and T-C = 1023 K. P-D and eta for the SEL converter at the optimum T-C (873 K) are 3.74 We/cm(2) and 17.0%, respectively, and they decrease to 1.61 W-e/cm(2) and 10.4%, respectively, as TG increases by 150 to 1023 K. The corresponding P-D and eta for the GC converter decrease from 2.56 W-e/cm(2) and 14.7% at T-C = 873 K to 2.38 W-e/cm(2) and 14.3%, respectively, at T-C = 1023 K. Those for the GE converter decrease from 2.18 W-e/cm(2) and 12.9% at T-C = 873 K to 1.56 W-e/cm(2) and 11.0%, respectively, at T-C = 1023 K. For the GEL converter, P-D and eta decrease from 1.86 W-e/cm(2) and 11.2% at T-C = 873 K to 1.44 W-e/cm(2) and 9.7%, respectively, at T-C = 1023 K. At T-E = 1673 K and T-C = 1023 K, the converter with a GC has the lowest V-B and operates at the highest P-D (2.38 W-e/cm(2)) and eta (14.3%). (C) 2003 Elsevier Ltd. All rights reserved.