Discussed are: (i) experimental results for the variation of the oxygen content, the electrical conductivity and the iron tracer diffusion in fayalite, Fe 2 SiO 4 , (ii) the relationship between point defects, the deviation from stoichiometry, δ, in Fe 3 - δ O 4 and the iron tracer diffusion, (iii) the deviation from stoichiometry and the cation tracer diffusion in iron-based spinels of the type (Me x Fe 1 - x ) 3 - δ O 4 and (iv) the influence of near-boundary regions in polycrystalline Fe 3 - δ O 4 and Cu 2 - δ O on the overall variation of the oxygen content as a function of the oxygen partial pressure.