When the inter‐electrode spacing is very small in an alkali‐metal vapor diode, plasma heat transfer cannot be neglected. An efficiency analysis very similar to that of Schock includes this effect and compares it to that due to radiation loss. In the analysis the heat‐transfer coefficienth[W/(cm)2(°K)] is given values 0, 0.005, and 0.01. This range is based on the experimental values ofhobtained in potassium and cesium diodes. The greatest effect ofhon efficiency is found theoretically to occur when the electron‐emitter work function or output voltage is larger than the optimum value for a particular temperature.