The mixed conducting solid solution 0.75Bi2O3−0.25Tb4O7 (BT40) was studied by impedance techniques using ionically blocking electrodes. These measurements confirmed the p-type electronic conductivity suggested in literature. In air at temperatures between 600 and 900 K the ionic transference number ranges from 0.28 to 0.70. The total conductivity at 833 K in air is 0.93 Ω−1 m−1. The activation enthalpy for the electronic conductivity varies from 77 kJ/mol in pure O2 to 83 kJ/mol in an ambient with PO2=2×10−4 atm. The electronic conductivity seems to be a function of the sample geometry and increases with decreasing cross section to height ratio. The activation enthalpy for the ionic conductivity being 110 kJ/mol, is independent of the PO2 and does not depend on the sample geometry.