The perovskite-related phases Sr6Ta2O11, Sr-5.92,Ta2.08O11.12(V-o(s))(0.88), Sr5.46Ta2.54O11.81(V-o(s))(0.19), Ba6NbO11(V-o(s))(1), Ba4Ca2Nb2O11, where V-o(s) stands for 'oxygen structural vacancies', have been studied by thermogravimetry, AC-conductivity and IR and Raman spectroscopy. A correlation between water uptake and nominal V-o(s) concentration was established, but the nature of the alkaline earth metal atom influences the oxygen sublattice: Ba6Nb2O11 (V-o(s)), has zero water uptake. The relationship between short-range disorder and water uptake is clear: only compounds having a strong oxygen and cation lattice disorder exhibit maximal water uptake: Sr6Ta2O11 and Ba4Ca2Nb2O11. The protonic species change during cooling-heating.