Chemical diffusivity of a mixed ionic-electronic conductor compound is often measured by monitoring the relaxation of electrical conductivity with time under a component's chemical potential gradient. Conductivity relaxations were systematically examined on undoped polycrystal BaTiO3, acceptor-doped polycrystal BaTiO3, donor-doped polycrystal BaTiO3, and undoped single crystal TiO2 upon oxidation and reduction, and on proton-conducting, acceptor-doped polycrystal SrCeO3 and BaCeO3 upon oxidation/reduction or hydration/dehydration at elevated temperatures. It has been found that there are three patterns of relaxation: single-fold (single relaxation time) monotonic, twofold (two relaxation times) monotonic and twofold non-monotonic relaxation. The inner working for each pattern is discussed and a general solution to the relaxations is presented in closed form, together with the chemical diffusivities as evaluated thereby. (C) 2009 Elsevier B.V. All rights reserved.