This paper shows the importance of including the activation losses in a model designed to accurately predict the voltage-current relationship of a modem solid oxide fuel cell. The governing equation for voltage drop due to activation losses, the Butler-Volmer equation, is discussed and the derivation and applicability of several approximations is investigated. Finally, the assumptions required to combine the separate anode and cathode activation loss calculations into a single-term equation are claritied. (c) 2005 Elsevier B.V. All rights reserved.