The conductivity ceiling phenomena of O2- ions in stabilized zirconia are studied in terms of the path probability method and Monte Carlo simulation. It is shown that there exist two kinds of possible mechanisms. One of them is due to an order-disorder transition of O2-ions, which is caused by the repulsive interaction between O2- ions, and the other is owing to a percolation transition of dopant cations network. It is proposed that the YSZ case originates in the percolation transition. (c) 2006 Elsevier B.V. All rights reserved.