Internal friction (IF) experiments were performed on the systems ZrO2 (8-12 mol%)-Y2O3, and ZrO2 (9-12 mol%)-Sc2O3. The following features were found: (i) an IF peak around 500 K in both systems which is assigned to the hopping of trapped oxygen vacancies (DeltaH=1.2-1.3 eV); (ii) in polycrystalline materials a high temperature damping background, which is related to cation diffusion; (iii) an order/disorder phase transition at 1450 K in ZrO2-12 mol%Y2O3; and (iv) in ZrO2-Sc2O3 a ferroelastic<---->ferroelectric phase transition at 750 K from the rhombohedral to the cubic phase. Cation diffusion experiments in ZrO2-Sc2O3 revealed an activation enthalpy for Zr diffusion of 5.3 eV indicating that short- and long-range cation transport is governed by one type of cation diffusion process. (C) 2004 Elsevier B.V. All rights reserved.