Annealing at 1000 degreesC reduces the electrical conductivity of 8 mol% yttria-stabilised zirconia (8YSZ), known as a good performing material for Solid Oxide Fuel Cells (SOFC) electrolytes. This decrease in electrical conductivity by ageing for 1000 h was not observed for doped zirconia with higher yttria contents. High-resolution XRD, neutron-diffraction and STEM were performed to get information of possible changes in the microstructure. Both, the electrical measurements and structural investigations of yttria-stabilised zirconia have been correlated. It is explained that the degradation was not caused by the phase change that occurs in samples with lower dopant content. The ageing effect of the electrical properties is interpreted by the presence of different defect associates present before annealing in all the doped zirconia specimens, or rather the transformation within these associates. This transformation also affects the observed structural change. A confirmation of this model of transformation is given in part II, wherein the scandia-stabilised system has been studied. (C) 2004 Elsevier B.V. All rights reserved.