AC impedance measurements were carried out under pressurized conditions by using a Ni-Y2O3-stabilized zirconia (YSZ)/YSZ half cell in order to investigate anodic polarization at high-pressure conditions. AC impedance spectra were measured at 900 and 1000 degreesC in H-2-H2O system with a constant H-2/H2O ratio, or a constant partial pressure of H-2 or H2O for different total pressures of 1 to 10 atm. At high pressures, the resistance characterized by the semicircle at high frequency was lowered, whereas that at low frequency was raised. A model based on one-dimensional diffusion was developed to estimate concentration polarization based on the impedance measurements, and activation polarization was evaluated using a linear current-potential relation derived from the Butler-Volmer equation. The activation overvoltage was at most 40 mV at 10 mA/cm(2), irrespective of the total pressure. Concentration polarization was computed to increase as the total pressure was raised, whereas it was almost constant for temperature change. Large voltage drop at small current densities was calculated for the system with low partial pressure of oxygen. (C) 2004 Elsevier B.V. All rights reserved.