Hydration/dehydration kinetics of proton conducting oxides has long been believed to be due to the chemical diffusion of H2O (or ambipolar diffusion of H+ and O2-), causing defect-structure-sensitive properties, e.g., electrical conductivity and mass, of the oxides to relax single-fold. Thus, a mass relaxation with a change of water activity used to be regarded as a measure of the equilibrium water solubility. It has recently been observed, however, that the conductivity and mass relax are not single-fold, but two-fold upon hydration or dehydration in usual oxidizing atmospheres, indicating that the above does not proceed via single-fold chemical diffusion of H2O, but via decoupled two-fold chemical diffusion of H (or ambipolar diffusion of H+-H+) and 0 (or ambipolar diffusion of O2- + 2 h(+)). In this paper, we will show theoretically and demonstrate experimentally that the two-fold kinetics asymptotically converges to the single-fold as the hole transference number diminishes to zero with decreasing oxygen activity in the ambient. Its implications are discussed particularly concerning the hydration equilibrium constant or equilibrium water solubility as determined from the mass relaxation. (C) 2013 Elsevier B.V. All rights reserved.