Partial electronic conductivity and total conductivity have been determined by Hebb-Wagner polarization method and a.c. impedance spectroscopy, respectively, on bilayer electrolyte Zr0.84Y0.16O1.92(YSZ)/Ce0.9Gd0.1O1.95(GDC) with thickness ratios 10(-3)/1 and 10(-4)/1 at 800 degrees, 900 and 1000 degrees C, respectively. While their ionic conductivities remain close to that of GDC, the electronic conductivities are suppressed the more from that of GDC towards that of YSZ the higher the thickness ratio, as expected. Even when the GDC-side is exposed to reducing atmosphere, the electronic conductivity is also suppressed, but to a less extent. It is suggested that oxygen activity distribution is discontinuous across the YSZ/GDC interface under ion-blocking condition, refuting the "continuity hypothesis" that has been usually adopted in calculating the oxygen activity distribution across a multilayer of mixed conductor oxides. The electrolytic domain widths of the bilayer electrolyte are reported depending on temperature, thickness ratio and direction of oxygen activity gradient imposed. (C) 2011 Elsevier B.V. All rights reserved.