Aluminium is an important constituent of advanced structural and functional ceramics, as well as of geological materials in the upper mantle. Aluminium has only one single stable isotope and radioisotopes which are difficult to apply routinely in radiotracer diffusion experiments. Therefore, it is a methodological challenge to measure aluminium diffusivities. Our novel approach consists in using the long-life radioactive isotope Al-26 as a pseudo-stable isotope whose depth distribution can be analysed with Secondary Ion Mass Spectrometry after a diffusion experiment. For our demonstration we chose mullite (Al4+2xSi2-2xO10-x, 0.2 < x < 0.5) which is an important candidate for advanced structural and functional ceramics with some outstanding properties: low thermal expansion, low thermal conductivity, and excellent creep resistance. The method is universally applicable to the huge family of aluminium containing oxides. (c) 2005 Elsevier B.V. All rights reserved.