Terbia (TbO1.75 or Tb4O7) has been dissolved into yttria [10 and 8 mole percent (m/o)] stabilized zirconia (Y10SZ and Y8SZ) to form mixed conducting yttria-stabilized zirconia-terbia (Y10ZTb and Y8ZTb) solid solutions, with terbia (TbO1.75) concentrations as high as 50 m/o. The total electrical conductivity between 300 and 800 degrees C in 1 atm oxygen, air, and 10(-5) atm P-O2 has been determined from impedance spectroscopy measurements. Electromotive force cell and oxygen ion blocking electrode techniques have been used to determine oxygen ion transference numbers and p-type conductivities in air and 1 atm P-O2 at temperatures between 600 and 833 degrees C. The oxygen-ion conductivity is more predominant in Y10ZTb and Y8ZTb containing 20 m/o terbia (TbO1.75) in air at temperatures >650 degrees C, whereas the p-type electronic conductivity is more predominant in Y10ZTb and Y8ZTb containing 30 m/o terbia. The calculated oxygen ion and electron hole mobilities of Y8ZTb30 at 800 degrees C in air are 7.8 X 10(-7) cm(2)/V s and 2.8 X 10(-5) cm(2)/V s, respectively. The activation energies and the calculated electron hole mobility for p-type conductivity support a small polaron mechanism in these YZTb-mixed conductors.