Firstly, the production of solid bulk specimens of the proton conductor Ba3Ca1.18Nb1.82O9-delta (BCN18) of known water content up to [H-.] = n(H)/n(Ba) = 0.16 is described. Secondly, measuring the length change of such samples versus water content [H-.] demonstrated that the sample lengths increased linearly with a slope of (Delta l/l)/[H-.] = (2.13+/-0.07) x 10(-2). Thirdly, the density of bulk samples was found to decrease linearly with water content [H-.]. This decrease was in good agreement with the above values derived from the length change. Fourthly, high temperature dilatometry showed that samples reach the thermodynamic solubility values in water vapor atmospheres only at temperatures above 700 degrees C. Two time scales were observed for the time-dependence of the elongation upon exposure to water vapor. A fast process occurred within minutes above 700 degrees C, a slow one took several hours. The fast one was identified as the chemical diffusion of the diffusion pair H-. and V-0(..) which is required for the water uptake and loss of BCN18. The chemical diffusivity of water is described by the parameters D-0=(0.022+/-0.002) cm(2)/s and Delta H*=(0.79+/-0.05) eV.