Layered LnBaCo(2)O(5+delta) (La = Nd, Sm) with the cation-ordered double perovskite structure were synthesized by the solid-state reaction route and characterized by X-ray diffraction, thermogravimetric analysis and dilatometry. For NdBaCo2O5.73 and SmBaCo2O5.61 equilibrated with atmospheric oxygen at low temperatures, tetragonal and orthorhombic polymorphs were found to form, respectively. The oxygen content at 300-1300 K decreases with decreasing rare-earth cation size, whilst 8 variations and chemical contribution to the apparent thermal expansion in air are substantially lower compared to the disordered (Ln, A)CoO3-delta (A = Ca, Sr) analogues. The average thermal expansion coefficients are 23.1 x 10(-6) K-1 for NdBaCo2O5+delta and 20.8 x 10(-6) K-1 for SmBaCo2O5+delta at 300-1370 K and atmospheric oxygen pressure. These values are comparable to those of Bi2O3-based ionic conductors, but are incompatible with common electrolytes such as stabilized zirconia or doped ceria. The oxygen partial pressure dependencies of the total conductivity and Seebeck coefficient, studied in the P(O-2) range from 10(-10) to 1 atm, confirm predominant p-type electronic conductivity. (C) 2010 Elsevier Ltd. All rights reserved.