The mass transport characteristics of sodium in cathode-side electrodes strongly influence the overall output performance of the alkali metal thermal-to-electric converter (AMTEC). At temperatures above 1100 K the rapid degradation of molybdenum electrode performance with time has been reported. On the other hand, a high initial performance may be maintained for much longer at lower operating temperatures. In this paper, measured performance of the molybdenum electrode in the temperature range of 900 to 1050 K and time dependence of the performance are discussed. The sodium exposure test cell (SETC) was used to measure the electrode morphological dimensionless factor, G, which can be calculated from the limiting current in the current-voltage curve. G was observed to become approximately 20 after 150 h of operation at 904 K. The analysis based on the experimental results shows that the molybdenum electrode operating at 900 K has a predicted output power density of 0.12 W/cm(2) and its performance is about 50% higher than that of the conventional titanium nitride electrode.