The effect of Ni substitution on the crystal chemistry, thermal and electrochemical properties and cat, lytic activity for oxygen reduction reaction of the layered NdBaCo2-xNixO5+delta perovskite oxides has been investigated for 0 <= x <= 0.6. The oxygen content (5 + delta) and oxidation state of the (Co, Ni) ions in the air-synthesized NdBaCo2-xNixO5+delta samples decrease with increasing Ni content, accompanied by a structural transition from tetragonal (0 <= x <= 0.4) to orthorhombic (x = 0.6). Similarly, the thermal expansion coefficient (TEC) and electrical conductivity also decrease with increasing Ni content. The x = 0.2 and 0.4 samples exhibit slightly improved performance as cathodes in single cell solid oxide fuel cell (SOFC) compared to the x = 0 sample, which is in accordance with the ac-impedance data. Among the samples studied, the x = 0.4 sample exhibits a combination of low thermal expansion and high catalytic activity for the oxygen reduction reaction in SOFC. (C) 2009 Elsevier Ltd. All rights reserved.