The mixed perovskites BaCa(1+x)/3Nb(2-x)/3O3-x/2 were investigated for x = 0, 0.06, 0.12 and 0.18. The latter compound is the widely studied ceramic BCN18 which has served as a prototypical high temperature proton conductor in recent years. The ceramics with x = 0.06 and 0.12 show a much lower solubility for water vapor. A linear increase in lattice parameter was observed when going from x = 0 to 0.18. Density measurements supported the model of vacancies on the O-sublattice in the dry state. The volume of such a vacancy is close to that required by an O-atom.