During extended testing at the Jet Propulsion Laboratory and the Air Force Research Laboratory, it has been found that maximum power output of the PX-3A AMTEC cell decreases with time. Starting with a peak maximum power output of 2.45 W it decreases almost continuously to 1.27 W at the end of 18 000 h. In this paper, the factors contributing to this power loss have been investigated. The cell uses a beta " -alumina solid electrolyte (BASE) whose material degrades with time due to changes in the chemical and thermal conditions within the cell during operation. The analysis shows that the beta " -alumina degradation manifests itself as an increase in its ionic resistance which reduces power output. The beta " -alumina is responsible most of the power degradation in the first 7000 h of operation. Thereafter, though beta " -alumina degradation continues to cause power loss, other components and their materials in the cell also contribute to power loss. Some suggestions are made that will help reduce the rate of power degradation and extend the useful and functional time of the cell which primarily deal with reducing the chemical contamination of the BASE, use of beta -alumina instead of beta " -alumina and keeping the electrode current density below a certain critical value. (C) 2001 Elsevier Science B.V. All rights reserved.