The phase diagram of the Bi2O3-Er2O3 system was investigated. A monophasic f c c structure was stabilized for samples containing 17.5–45.5 mol% Er2O3. Above and below this concentration range polyphasic regions appear. The f c c phase showed high oxygen ion conduction. The ionic transference number is equal to one for specimens containing 30 mol% Er2O3 or less, while an electronic component is introduced at low temperatures for specimens containing 40–60 mol% Er2O3. Between 673 K and 873 K a maximum in the conductivity was found at 20 mol% Er2O3. (Bi2O3)0.8.(Er2O3)0.20 is found to be the best oxygen ion conductor so far known. The conductivity at 773 K and 973 K is 2.3 Ω−1m−1 and 37 Ω−1 m−1 respectively. These values are 2–3 times higher than the best oxygen ion conductor reported for substituted Bi2O3 systems and 50–100 times higher than those of stabilized zirconia (ZrO2)0.915(Y2O3)0.085 at corresponding temperatures.