A thermodynamic analysis has been performed to compare the theoretical performance of ammonia fed solid oxide fuel cells (SOFCs) based on proton-conducting electrolyte (SOFC-H) and oxygen ion-conducting electrolyte (SOFC-O). It is found that the ammonia fed SOFC-H is superior to SOFC-O in terms of theoretical maximum efficiency. For example, at a fuel utilization of 80% and an oxygen utilization of 20%, the efficiency of ammonia fed SOFC-H is 11% higher than that of SOFC-O. The difference between SOFC-H and SOFC-O becomes more significant at higher fuel utilizations and higher temperatures. This is because an SOFC-H has a higher hydrogen partial pressure and a lower steam partial pressure than an SOFC-O. In addition, an increase in oxygen utilization is found to increase the efficiency of ammonia fed SOFCs due to an increase in oxygen molar fraction and a reduction in steam molar fraction. With further development of new ceramics with high proton conductivity and effective fabrication of thin film electrolyte, the SOFC based on proton-conducting electrolyte is expected to be a promising approach to convert ammonia fuel into electricity. (C) 2008 Elsevier B.V. All rights reserved.