The structure, conductivity and water uptake of the oxygen-deficient compound Ba4Ca2Nb2O11 have been investigated. Ba4Ca2Nb2O11 is found to crystallize in the cryolite structure with a = 8.4449 Angstrom. The compound may be fully hydrated up to maximum water content approximately 1 mol H2O per mol Ba4Ca2Nb2O11, this process is accompanied by a reversible phase transformation due to formation of hydrate-like compound. In wet air, the temperature dependence of total conductivity shows a different behavior because of change in proton transport mechanism: small local maximum as a result of phase transformation (T = 350 degreesC), linear relationship with activation energy 0.55 eV at the temperature range 350-230 degreesC and 1.02 eV at lower temperature. A comparative analysis with Ba3Ca1.18Nb1.18O9-delta was made. (C) 2003 Elsevier B.V. All rights reserved.