Hydration or dehydration of a proton conducting oxide has been understood to be due to chemical diffusion of H2O. Hence, mass relaxation of the oxide has often been employed to determine the chemical diffusivity and saturation solubility of H2O. This paper shows that this approach is not correct. Applying thermogravimetric methods at 973 and 1073 K to BaCe(0.95)yb(0.05)O(2.975) as model system, the mass relaxes twofold (i.e., with two different chemical diffusivities for H and O) like the electrical conductivity, but monotonically unlike the conductivity which relaxes non-monotonically. The correlation of the mass relaxation with the conductivity relaxation is established. The closed-form solution for the mass relaxation is presented in comparison with that for the conductivity relaxation and its implication is discussed in connection to water saturation. (C) 2009 Elsevier B.V. All rights reserved.