Hydrated mobates and tantalates of alkaline-earth metals Sr6Ta2O11, Sr6Nb2O11, and Ba4Ca2Nb2O11 with a perovskiterelated structure have been investigated by using wide-line H-1 nuclear magnetic resonance (NMR), thermogravimetry (TG), conductivity measurements, and IR spectroscopy.TG investigations show that these phases can incorporate up to similar to1 mol of water, which corresponds to the theoretical limit of hydration. A vide-line H-1-NMR study indicates that the proton signal consists of three components, because protons were present in the form of (i) relatively isolated OH groups, (ii) paired OH groups bound with the same metal atom, and (iii) closely spaced OH groups belonging to different octahedra and localized within a defect cluster. The niobates and the tantalates have different ratios of those groups that may be explained by different coordination tendencies of niobium and tantalum and as a consequence a different arrangement of oxygen vacancies. A dehydration scheme may be proposed from a comparison analysis of the TG and H-1-NMR data for partially hydrated samples. (C) 2003 Elsevier B.V. All rights reserved.