Results of recent tests of a Pluto/Express electric power generator ground demonstration were analyzed. The performance parameters of each of the eight ground demonstrations vapor anode, multitube alkali-metal thermal-to-electric conversion (AMTEC) cells, designated PX-3G, were analyzed and compared. The ground demonstration cells produced a total peak electric power of 27 W-e at a load voltage of 16 V when tested at hot and cold side temperatures of 1123 K and 553 K, respectively. The electric power output and terminal voltage of the individual cells, however, differed by as much as 25%, from 2.94 to 3.76 W-e, and from 1.73 to 2.21 V, respectively. These variations were attributed to differences among the cells in the values of: (a) the contact resistance of the BASE/electrode and of the electrode/current collector; (b) the leakage current between the anode and cathode electrodes through the metal-ceramic braze joint between the BASE tubes and the metal support plate; and (c) the charge-exchange polarization losses. Analysis of results suggested the existence of large electrical leakage currents in some of the PX-3G cells. The performance of the PX-3G cells was below that needed for meeting the Pluto/Express mission＇s electric power requirement.