The isothermal permeability of oxygen through sintered dense disks of bismuth oxide stabilized with 25 mol% erbia (BE25) has been studied at 610–810°C and oxygen pressures of 0.0001–1 atm. It is concluded that the permeating flux is rate limited both by solid state diffusion of electron holes and by the surface exchange reaction. From the results the p-type electronic conductivity of BE25 and the surface oxygen exchange rate have been evaluated. The value obtained for the latter shows excellent agreement with that obtained from 18O-16O isotope exchange reported previously. Using gold point electrodes it is demonstrated that the oxygen semipermeability flux in case of partial rate control by surface oxygen exchange leads to deviations from Nernst behaviour, though oxygen permeation measurements reveal that under the applied conditions BE25 remains a solid electrolyte with an ionic transference number close to unity.