Current efficiency of electrochemical hydrogen pumping from wet hydrogen gas through a SrCe0.95Yb0.05O3-alpha proton-conducting solid electrolyte was investigated at 900 degrees C, A decrease in current efficiency was observed at relatively high current density, The critical current density for the reduction of the efficiency decreased with increasing amount of water vapor in the anode hydrogen gas. Addition of a small amount of argon to the anode gas had no effect on the pumping characteristics. It was concluded that the lack of hydrogen and the increase in partial pressure of oxygen at the anode on sending high current could cause the partial hole conduction in the electrolyte and the decrease in current efficiency of the hydrogen pump. (C) 2000 Elsevier Science B.V. All rights reserved.