Structural and electrical properties of the homogeneous solid solution series (TbxGd1−x)2Zr2O7+y (Ox1; O y < 0.25) were investigated. The defect structure and electrical properties were studied by thermogravimetric analysis and electrical conductivity and ionic transport number measurements as a function of temperature and oxygen partial pressure. The (p-type) electronic conductivity was found to arise from a small polaron hopping mechanism on the Tb-sublattice. The charge carrier concentration can be correlated with the Tb4+ concentration. The maximum fraction of Tb4+ ions on the Tb-sublattice was found to be 0.25 and is hardly dependent on the composition (x). Charge carrier mobilities are in the range at 700°C. Ionic conductivities up to 7 × 10−1 ω−1m−1 at 700°C are found. The conductivity can well be described with a previous developed model, that relates the effects of pyrochlore order in the fluorite lattice with the conductivity parameters.