The performance of thermionic converters with planar, macro-grooved, molybdenum electrodes was investigated experimentally as a function of the cesium pressure, P-Cs, in two configurations: (a) grooved emitter (or cathode) and smooth collector (or anode) and (b) smooth emitter and grooved collector. The results were compared with those of smooth electrodes converter tested in the same conditions. The emitter and collector temperatures were 1573 and 1073 K, respectively, while P-Cs was varied from 10 to 240 Pa. The concentric macro-grooves were 0.5 mm deep and 0.5 mm wide and 1.0 mm apart. The geometrical surface area of the grooved electrode was similar to 40% larger than the smooth electrode (2 cm(2)). The inter-electrode gap for all three converters was 1.5 mm. The electric power output of the grooved emitter converter was higher than that of the smooth electrodes converter at all values of cesium pressure. It was more than a factor of two higher at P-Cs of 57 Pa, decreasing to only 5% at 240 Pa. The peak electric power for the grooved emitter converter (0.8 W) occurred at P-Cs = 120 Pa, while that of the smooth electrodes converter (0.65 W) occurred at a higher P-Cs = 150 Pa. The electric power output of the grooved collector converter was consistently 40% higher than that of the smooth electrodes converter, which is the same increase as in the surface area of the grooved collector.