The surface oxygen exchange coefficient, ks, has been measured for the solid solution (Bi2O3)0.75(Er2O3)0.25 and (Bi2O3)0.6(Tb2O3)0.4 (abbreviated BE25 and BT40), using gas-phase 18O exchange techniques. The activation enth alpy of ks amounts to ΔE=110 kJ/molforBT40 and ΔE=130 kJ/molforBE25. The magnitude of ks for the purely ionic conducting BE25 is comparable with values obtained from electrode polarization (I−V) measurements (ΔE=140 kJ/mol.) The comparatively high ks values show a (PO2)n dependence on oxygen pressure with values of n close to 0.5, indicating surface control in the oxygen transport process. Bismuth oxide containing solid solutions show a large activity in the oxygen exchange reaction with the gas phase.