The effects of Cu substitution on the crystal chemistry, thermal and electrical properties, and catalytic activity for the oxygen reduction reaction of the layered LnBaCo(2-x)Cu(x)O(5+delta) (Ln = Nd and Gd, and 0 <= x <= 1.0) perovskite have been investigated. The LnBaCo(1.0)Cu(1.0)O(5+delta) compositions exhibit thermal expansion coefficients (TEC) of 16.4 x 10(-6)degrees C-1 for Ln = Nd and 14.5 x 10(-6)degrees C-1 for Ln = Gd in the range of 80-900 degrees C. The LnBaCo(2-x)Cu(x)O(5+delta) (x = 0.75 for Ln = Nd and x = 1.0 for Ln = Gd) cathodes exhibit low polarization resistance and fuel cell performance comparable to that of the well-known La1-xSrxCoO3-delta perovskite cathodes, but with an important advantage of lower TEC. The better performance of these Cu-substituted samples is partly due to the improved chemical stability with the La0.8Sr0.2Ga0.8Mg0.2O2.8 electrolyte at high temperatures (Ln = Gd) and a better matching of the TEC value with those of the standard electrolytes. With an electrolyte-supported single cell configuration (thickness = 0.50 mm), NdBaCo1.25Cu0.75O5+delta and GdBaCo1.0Cu1.0O5+delta exhibit higher maximum power densities of, respectively, 772 and 634 mW cm(-2) at 800 degrees C compared to that of the Cu-free (x = 0) sample (658 and 595 mW cm(-2)). (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3527006] All rights reserved.