The effect of Fe substitution for Co on the crystal chemistry, thermal and electrical properties, and catalytic activity for oxygen reduction reaction of the layered LnBaCo(2-x)Fe(x)O(5+delta) (Ln = Nd and Gd) perovskite has been investigated. The air-synthesized LnBaCo(2-x)Fe(x)O(5+delta) samples exhibit structural change with increasing Fe content from tetragonal (0 <= x <= 1) to cubic (1.5 <= x <= 2) for the Ln = Nd system and from orthorhombic (x = 0) to tetragonal (0.5 <= x <= 1) for the Ln = Gd system. The thermal expansion coefficient (TEC) and electrical conductivity decrease with increasing Fe content in LnBaCo(2-x)Fe(x)O(5+delta). While the substitution of a small amount of Fe (x = 0.5) for Co leads to slightly improved performance in solid oxide fuel cells (SOFC). larger Fe contents (x >= 1.0) deteriorate the fuel cell performance. In the Ln = Gd system, the better performance of the x = 0.5 sample is partly due to the improved chemical stability with the LSGM electrolyte at high temperatures. With an acceptable electrical conductivity of >100 S cm(-1) at 800 degrees C. the x=0.5 sample in the LnBaCo(x-2)Fe(x)O(5+delta) (Ln = Nd and Gd) system offers promising mixed oxide-ion and electronic conducting (MIEC) properties. (C) 2010 Elsevier B.V. All rights reserved.