In the system ZrO2-In2O3, the In2O3-doped ZrO2 phases (cubic, tetragonal and t’) exhibit high ionic conductivity and the ZrO2-doped In2O3 high electronic conductivity. These phases are in thermodynamic equilibrium at high temperatures. The ionic conductivity of ZrO2 depends on the crystal symmetry having the same In2O3 concentration. At 1000 degrees C, the highest conductivities were obtained for cubic ZrO2 doped with 25 mol% InO1.5. At lower concentrations, the ionic conductivity of cubic-ZrO2 decreases due to a first-order phase transformation to the tetragonal (t’) form. Single-phase In2O3 doped with ZrO2 is an n-type electronic conductor with a conductivity of up to 7 X 10(4) S/m in air. Point defect models for electronic conduction in In2O3 doped with ZrO2 are discussed. Two maxima in the electronic conductivity have been found : one in the two-phase region and one in the InO1.5 single phase region. In the heterogeneous two-phase material cubic-ZrO2 + InO1.5, the electronic conductivity increases abruptly up to 10(4) S/m with increasing InO1.5 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.