Electrical conductivity measurements and hydrogen-oxygen fuel cell experiments were carried out at 600-800 degreesC using proton-conducting Yb-doped SrCeO3. The total conductivity increased with P-O2 higher than 10(-5) atm and decreased with respect to P-H2O, suggesting that electron hole formation was suppressed in the presence of water. In fuel cell tests, the introduction of water to the oxygen cathode resulted in a lower OCV due to the converse hydrogen potential, but a higher ionic transport number in agreement with the conductivity measurements. The introduction of water to the hydrogen anode did not affect the OCV but improved (lowered) the area-specific resistance (ASR), probably by promotion of the uptake of protons through hydroxyl insertion. (C) 2003 Elsevier B.V. All right reserved.