We have measured the oxygen self-diffusivity in mullite ceramics via gas/solid exchange experiments in an atmosphere enriched in the rare stable isotope O-18. The resulting O-18 isotope depth distributions were measured by SIMS depth profiling. Our investigation showed that grain boundary diffusion coefficients are about five orders of magnitude higher than lattice diffusivities. While the pre-exponential factors differ by about two orders of magnitude, the activation enthalpies in the volume (433 +/- 21 kJ/mol) and in the grain boundaries (363 +/- 25 kJ/mol) are surprisingly close to each other. (C) 2002 Elsevier Science B.V. All rights reserved.