The performance of a hybrid system of solid oxide fuel cells with different electrolytes, i.e., an oxygen-ion conducting electrolyte (SOFC-O(2-)) and a proton-conducting electrolyte (SOFC-H(+)) is evaluated in this study. Due to an internal reforming operation, SOFC-O(2-) can produce electrical power as well as high-temperature exhaust gas containing remaining fuel, i.e., H(2) and CO that can be used for SOFC-H(+) operation. The remaining CO can further react with H(2)O via water gas-shift reaction to produce more H(2) within SOFC-H(+) and thus, the possibility of carbon formation in SOFC-H(+) can be eliminated and overall system efficiency can be improved. The simulation results show that the performance of the SOFC-O(2-)-SOFC-H(+) system provides a higher efficiency (54.11%) compared with the use of a single SOFC. Further, the SOFC hybrid system performance is investigated with respect to important operating conditions, such as temperature, pressure, degree of pre-reforming, inlet fuel velocity, and cell voltage. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.