The phase diagram of the Bi2O3-Dy2O3 system was investigated. A monophasicfcc structure was stabilized for samples containing 28.5–50.0 mole percent (m/o) Dy2O3. Above and below this concentration range polyphasic regionsappear. The fcc phase showed high oxygen ion conduction. The ionictransference number is equal to one for specimens containing 28.5–40.0 m/o Dy2O3, whereas an electronic component is introduced at low temperaturesfor specimens containing 50.0 m/o Dy2O3. The conductivity of (Bi2O3)0.715(Dy2O3)0.285 is 0.71 Ω–1m–1 and 14.4 Ω–1m–1 at 773 and 973 K, respectively.Relations were found between the ionic radius, the conductivity, and the minimumconcentration of lanthanide necessary to stabilize the fcc phase. It is concludedthat the highest ionic conductivity will be found in the system Bi2O3-Er2O3or Bi2O3-Tm2O3. From a study of relations between the activation energy, log σ0 and the composition it is concluded that two conductivity mechanismsplay a role.