This paper studies the thermodynamic aspects of direct ammonia-fed solid oxide fuel cells (DA-SOFC). In this regard, anode-supported oxygen ion-conducting solid oxide fuel cells (SOFC-O) and proton-conducting solid oxide fuel cells (SOFC-H) fuelled with ammonia are compared on the basis of their thermodynamic and electrochemical performances at open and closed circuit conditions. Based on the thermodynamic equilibrium, full conversion of ammonia is attained at a temperature of 700 K and atmospheric pressure. From the results of this study, it is found that the overall performance of the fuel cell is largely dependent on critical factors such as the operating temperature and the properties of the electrolyte and electrodes layers. On average, the peak power density of ammonia-fed SOFC-H is 20-30% higher than its SOFC-O counterpart under most examined conditions. This is mainly attributed to the higher concentration of hydrogen at the anode of SOFC-H in all cases. (C) 2011 Elsevier B.V. All rights reserved.