In this paper, we report the synthesis, crystal structure and electrical transport properties of new K-doped Ba3CaNb2O9 (BCN) and investigate their chemical stability in H2O and pure CO2 at elevated temperature. The powder X-ray diffraction (PXRD) of Ba2.5K0.5CaNb2O9-delta, Ba2.25K0.75CaNb2O9-delta, Ba2KCaNb2O9-delta, and Ba1.73K1.25CaNb2O9-delta showed the formation of a single-phase double perovskite (A(3)BB'O-2(9))-like cell with a lattice constant of a similar to 2a(p) (where a(p) is a simple perovskite cell of similar to 4 angstrom). Perovskite-like structure was found to be retained after treating with CO2 at 700 degrees C and also after boiling H2O for 120 h. The lattice constant of CO2 and H2O treated samples was found to be comparable to that of the corresponding as-prepared compound. The total electrical conductivity of all the investigated K-doped BCN increases with increasing K content in BCN in various atmospheres, including air, dry H-2, wet N-2 and wet H-2. The electrical conductivity in dry and wet H-2 atmospheres was found to be higher than that of air in the temperature range of 300-700 C, while in wet N-2 a slightly lower value was observed. Among the compounds investigated in the present study Ba1.75K1.25CaNb2O9-delta showed the highest total electrical conductivity of 1 x 10(-3) S/cm in dry H-2 at 700 degrees C with an activation energy of 1.28 eV in the temperature range of 300-700 degrees C. Crown Copyright (C) 2010 Published by Elsevier B.V. All rights reserved.