The present paper provides a thermodynamic foundation and interpretation of the concept of exergetic efficiency. The interpretation focuses on solid oxide fuel cell behavior, specifically the impedance of SOFCs as an indicator of exergetic efficiency. Subsequently, that concept is extended to the consideration of changes in response during long-term operation, resulting in a discussion of the possible use of exergetic efficiency with the interpretation in terms of impedance as a method of prognosis of expected performance based on observed impedance changes. Copyright (C) 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.