A two-dimensional electric model of vapour anode, multitube alkali-metal thermal-to-electric conversion (AMTEC) cells was developed. These cells are being developed to power the Pluto/Kuiper Express and Europa Orbiter spacecrafts, at NASA which are scheduled for launch early in the next century. Model results of several cells tested at the Air Force Research Laboratory showed that electric losses in the current collector networks and the connecting leads were negligible. The charge-exchange polarization/concentration losses in the TiN electrodes were the major losses, amounting to 25-50% of the cell＇s total theoretical power, while the contact losses and the beta "-alumina solid electrolyte ionic losses amounted to less than 16% of the cell theoretical power. Results also showed that a cell with advanced Rh2W electrodes could have delivered from 14% to 25% more electrical power.