Thin films of Ba3Ca1.18Nb1.82O8.73, prepared in a sol–gel process by multiple dip-coating on silicon wafers, were charged with hydrogen by dissociative water absorption at definite values of water vapour pressures and temperatures. The hydrogen content was determined using nuclear resonance reaction analysis. From the resulting water vapour pressure/hydrogen composition isotherms the absorption enthalpies and the absorption entropy were calculated in the framework of a two site model, based on Fermi–Dirac statistics. On Ba3Ca1.18Nb1.82O8.73 also impedance spectroscopy was performed yielding the bulk conductivity. From these data in combination with proton transport numbers and the thermodynamic results as noted above the proton diffusion coefficient could be evaluated.