In the present work the operation of a SOFC based on a co-ionic electrolyte is considered. The special case of ion transport in a co-ionic electrolyte is analyzed. It is shown that partial ion currents do not correspond to ion transfer numbers, a partial proton current being higher than a proton transfer number under the working conditions of the SOFC. It is shown that both fuel utilization and electric efficiency are higher when the proton transfer number is higher. It is established that the maximum achievable efficiency of a hydrogen-fed SOFC based on the co-ionic electrolyte with t(H) = 0.5 is 0.78 at 900 K whereas the efficiency is 0.62 when the SOFC works at 70% of its maximum power. (C) 2004 Elsevier B.V. All rights reserved.