Ion conducting yttria-stabilized tetragonal zirconia (3YTZ) and Er 3+ -, Nd 3+-and Hf 4+ -doped 3YTZ ceramics were prepared and studied in this work. It is noted that dopant-induced structural effects, associated with free lattice volume for bulk conduction, may still be dominant over charge carrier density effects, even for variations of these latter by up to about 30%. In that way, dopant ion size-modified charge (oxygen vacancy) mobility varied to about +25% in Er 3+ -doped 3YTZ and about −45% in Nd 3+ -doped 3YTZ, with respect to original 3YTZ. Meanwhile, the behavior of grain-boundary electrical properties appeared to adapt well with Frenkel's space-charge model. In both bulk and grain-boundary cases, the electrical response of Hf 4+ -doped 3YTZ remained close to that from 3YTZ, a fact which is also discussed in this report.