The effects of mixed conduction on the open-circuit voltage of intermediate-temperature solid oxide fuel cells (SOFCs) based on SDC20 were studied in the temperature range of 723-973 K. Though the measured OCVs of a test cell using high activity electrodes agreed well with theoretical values calculated using the conductivity data at 973 K, they deviated from the theoretical ones at lower temperatures. The OCV was dependent on electrode activity and electrolyte thickness, and it was concluded that overpotential at the electrode/electrolyte interfaces affected the OCV because of internal short circuit. The ionic transference number of SDC 20 that is free from the effect of overpotential was obtained using a method derived by Liu et al. SDC20 exhibited high ionic transference number with suppressed electronic conduction at low temperatures with highly humidified fuel. These results indicate that the electronic conduction of SDC20 can be suppressed at intermediate temperatures with high humidification of fuel, which agreed well with the results from the theoretical consideration of the conductivity data. (C) 2004 Elsevier B.V. All rights reserved.