The Open-Access Journal for the Basic Principles of Diffusion Theory, Experiment and Application www.diffusion-fundamentals.org, ISSN 1862-4138; © 2005-2008 Abstract. The crystal structure of mayenite (ideal composition: Ca 12 Al 14 O 33) consists of a (disordered) calcium-aluminate framework comprising 32 of the 33 oxygen anions. It contains large open cages, 1/6 of them being filled randomly by the remaining "free" oxygen which is available for diffusion and very high ionic conductivities. This extra-framework oxygen could be substituted by nitrogen, opening up possibilities for a pure nitrogen ionic conductor. From high temperature neutron powder diffraction experiments, data analysis employing difference Fourier methods and anharmonic Debye-Waller factors and comparing pure O-mayenite with partly exchanged N/O-mayenite it is concluded that oxygen diffusion proceeds via jump-like processes, intimately coupled to relaxations of Ca and involving exchange of the "free" oxygen with framework oxygen, while nitrogen diffusion appears to proceed via an interstitial process.