The hydration behavior of the barium scandates with oxygen-deficient perovskite-related structures of tetragonal Ba2Sc2O5 and cubic BaScO3-delta (delta similar to 0.5) was studied. Both the tetragonal Ba2Sc2O5 and cubic BaScO3-delta absorbed water and formed Ba2Sc2O5 center dot 0.6OH(2)O and BaScO3-delta center dot 0.37H(2)O, respectively, during cooling in a humidified atmosphere. The hydration and the dehydration temperatures for the barium scandates were similar to 533 K and similar to 553 K, respectively; these temperatures were 30-70 K lower than that for Ba2In2O5. A distinct difference in the hydration behavior of the structurally different tetragonal Ba2Sc2O5 and cubic BaScO3-delta was not observed. The Ba2Sc2O5 shows protonic conduction below 673 K and p-type conduction above 773 K in a humidified atmosphere. (c) 2006 Elsevier B.V. All rights reserved.