The electrochemical properties of Ce0.8Sm0.2O1.9-delta (SDC20) at intermediate temperature (673-1073 K) were studied. The dependence of total conductivity on the partial pressure of oxygen [P(O-2)] showed that Ce4+ was partially reduced to Ce3+, and electronic conduction was seen at a low P(02) region at each temperature. This result suggested that it was difficult to avoid introducing electronic conduction even at 673 K. However, under open circuit and operating conditions, simulations revealed that it was possible to suppress electronic conduction by reducing the operating temperature and using high fuel humidification, resulting in a high P(O-2). Furthermore, the maximum efficiency increased with a decrease in temperature and an increase in P(O-2) on fuel side although the maximum power density decreased. For the cell with 10-mum-thick electrolyte and reversible electrodes, the total efficiency at 673-873 K showed almost the same value of 0.61 under the typical operating condition of 0.72 V when H-2 humidified at 293 K and air was used as fuel and oxidant, respectively. (C) 2004 Elsevier B.V. All rights reserved.