In the system ZrO 2 In 2 O 3 , the In 2 O 3 -doped ZrO 2 phases (cubic, tetragonal andt´) exhibit high ionic conductivity and the ZrO 2 -doped In 2 O 3 high electronic conductivity. These phases are in thermodynamic equilibrium at high temperatures. The ionic conductivity of ZrO 2 depends on the crystal symmetry having the same In 2 O 3 concentration. At 1000 °C, the highest conductivities were obtained for cubic ZrO 2 doped with 25 mol% InO 1.5 . At lower concentrations, the ionic conductivity of cubic-ZrO 2 decreases due to a first-order phase transformation to the tetragonal (t´) form. Single-phase In 2 O 3 doped with ZrO 2 is an n-type electronic conductor with a conductivity of up to 7 × 10 4 S/m in air. Point defect models for electronic conduction in In 2 O 3 doped with ZrO 2 are discussed. Two maxima in the electronic conductivity have been found: one in the two-phase region and one in the InO 1.5 single phase region. In the heterogeneous two-phase material cubic-ZrO 2 + InO 1.5 , the electronic conductivity increases abruptly up to 10 4 S/m with increasing InO 1.5 concentration. This material is a three-dimensional composite of ion- and electron-conducting phases. The origin of the maximum in electrical conductivity in the heterogeneous two-phase region is discussed.