From O-18-tracer diffusion, H-1-PFG-NMR and conductivity measurements, the diffusivities of protonic defects (OHo.) and oxygen ion vacancies (V-o(..)) in BaCeO3 single crystals doped with gadolinium and yttrium have been determined. Their dependence on the nature and concentration of the dopant, and the degree of hydration provide evidence for (i) a strong relaxation around oxygen ion vacancies which decreases with increasing vacancy concentration and (ii) the absence of significant defect interaction even for high dopant concentrations. The attempt frequencies are consistent with a proton diffusion mechanism assisted by the local oxygen dynamics. The thermodynamic data for the hydration reaction, including the water solubility, are in accordance with the conclusions drawn from the transport data. The hydration enthalpy is suggested to be determined not only by the formation enthalpy of oxygen ion vacancies but also by the basicity of the oxygen.