Ba(2)ln(2) (-) xMexO5 + delta (Me=V, Mo, W) solid solutions were investigated. Solid solution limits were found at x = 0.5 and x = 2/3 for V and Mo, W, respectively. Partial substitution for In led to the stabilisation of the Ba(2)ln(2)O(5) fast oxide ion conducting cubic form at lower temperatures. As for substitution by other cations with radius smaller than that of In3+, a decrease of the order-disorder transition temperature with the increase of substitution rate was observed. This was associated with a decrease in the conductivity with increasing cation substitution. Below 400 degrees C, an uptake of water was observed for all the materials, which also makes them promising as proton conductors. However, in this paper, only the electrical properties at high temperature are reported. (c) 2005 Elsevier B.V. All rights reserved.