The effects of collector heat pipe orientation upon the electrical and thermal performance of a planar thermionic converter were investigated. The high heat throughput of the converter must be carried away from the collector following electrical power production.The planar thermionic converter tested employs chemical-vapor-deposited rhenium on molybdenum electrodes, a separately heated two-phase cesium reservoir, and a radiantly coupled, electric emitter heater. The collector also functions as the evaporator end cap for the liquid sodium heat pipe. The converter fixture places the heat pipe in the reflux mode (evaporator below the condenser) and allows orientational changes of 15, 30, 45, and 60 degrees from the vertical.It was determined that the thermionic output performance is a relatively weak function of orientation angle in the range of 0 to 60°. As the emitter temperature increased, the effect of orientation on the converter performance increases. The trend is that maximum power and output voltage decreases as orientation increases.The performance of the heat pipe was also monitored at various emitter temperatures and orientations. The difference in the temperatures of the heat pipe evaporator and condenser increased as the angle of inclination from the vertical increased. Instabilities due to a lack of heat throughput and the inability to reprime the wick define a region of safe operation of the heat pipe with respect to heat input and tilt angle.