Ionic conduction and oxygen diffusion in Y2O3-doped zirconia are due to the mobility of oxygen vacancies. Vacancy association reactions with Y cations can be classified using the activation enthalpies DeltaH determined by various methods in dependence of the yttria content (3-24 mol%). DeltaH(O) for oxygen tracer diffusion is independent of the yttria content (average value DeltaH(O)=0.95 eV). The activation enthalpy of electrical conductivity, DeltaH(DC), rises from 1.05 to 1.5 eV, with increasing Y2O3 content, pointing to growing association effects. In dielectric loss experiments, slightly higher activation enthalpies were found. In internal friction experiments, localized jumps of vacancies bound to yttrium ions can be observed as Debye loss peaks. Here, the activation enthalpy DeltaH(ML) rises with increasing Y2O3 content from 0.9 to 1.6 eV The results are discussed with respect to the combined effect of migration, association and interaction of oxygen vacancies and stabilizer cations. (C) 2004 Elsevier B.V. All rights reserved.