The sintering, grain growth and ionic conductivities (especially the grain-boundary (GB) conductivity), of 8YSZ electrolytes with various silica levels (similar to 30 ppm, similar to 500 ppm and similar to 3000 ppm), doped with 1 at% transitional metal oxides (TMOs), have been systematically investigated by means of dilatometer, electron microscopy and impedance analyzer. It is confirmed that small additions of TMOs (i.e., Fe, Mn, Co or Ni) promote the densification and grain growth of both the pure and Si-containing 8YSZ. The effect of TMOs on the ionic conductivities could be negative or positive, relying on the type of TMOs, sintered density and impurity level. For the dense and pure 8YSZ (with similar to 30 ppm SiO2), the addition of 1 at% TMOs led to a reduction in grain interior (GI) conductivity by similar to 25-33% with little effect on the GB conduction. For the impure 8YSZ (with similar to 500 ppm or 3000 ppm SiO2), except for FeO1.5, the other TMOs (i.e., Mn, Co or Ni) are extremely detrimental to the total conductivity by significantly reducing the GB conduction. Moreover, it is also found that the GB conductivity of the impure 8YSZ doped with Co or Ni is less sensitive to sintering temperature. FeO1.5 showed a scavenging effect on SiO2 in the impure 8YSZ, which is specially beneficial to the total conductivity of samples with higher silica levels and/or sintered at relatively low temperatures. (C) 2009 Elsevier B.V. All rights reserved.