Discussed are : (i) experimental results for the variation of the oxygen content, the electrical conductivity and the iron tracer diffusion in fayalite, Fe2SiO4, (ii) the relationship between point defects, the deviation from stoichiometry, delta, in Fe3-deltaO4 and the iron tracer diffusion, (iii) the deviation from stoichiometry and the cation tracer diffusion in iron-based spinels of the type (MexFe1-x)(3-delta)O-4 and (iv) the influence of near-boundary regions in polycrystalline Fe3-deltaO4 and Cu2-deltaO on the overall variation of the oxygen content as a function of the oxygen partial pressure.