The effects of CuO additions on the grain interior and grain boundary resistivity in coated 2.5 mol% Y-TZP ceramics have been studied by using impedance spectroscopy. In addition, the effects of hydrothermal ageing on the conductivities of the ceramics were evaluated. The results showed that the grain interior and grain boundary resistivities increased with increasing CuO content. In particular, the grain boundary resistivity was observed to increase significantly in the doped ceramics, which could have been due to the formation of compounds, and resulted in lower conductivity. Impedance spectroscopy performed on the hydrothermally aged Y-TZPs revealed that the grain boundary regions were significantly affected, i.e. the grain boundary resistivities of the undoped and CuO-doped samples were increased by 65% and 45% respectively after ageing for 200 h. Thus, the indication is that the ageing-induced tetragonal to monoclinic phase transformation is closely associated with changes in the grain boundary regions.