Four-probe dc measurements were conducted on a polycrystalline material of composition Ba3Ca1.18Nb1.82O(9-delta), which showed that the conductivity changed linearly with time during the initial stages when the atmosphere was changed from dry air to wet air, or vice versa. In order to analyze this transient conductivity change with time, a model was developed assuming rapid surface reaction. The model was then used to fit the experimental traces of conductivity vs. time, and estimate the chemical diffusion coefficient of H2O, (D) over tilde H2O, over a range of temperatures from 400 to 900degreesC. The temperature dependence of the chemical diffusion coefficient could be described by an Arrhenius equation with an activation energy of similar to0.80 eV. The technique was also used to estimate the chemical diffusion coefficient of H2O in BaCe0.85Gd0.15O(3-delta).