The effect of Sr2+ substitution for Ba2+ on the crystal chemistry, oxygen content, thermal expansion, electrical conductivity, and catalytic activity for oxygen reduction reaction (ORR) of the double perovskite oxides GdBa1-xSrxCo2O5+delta has been investigated for 0 <= x <= 1.0. The GdBa1-xSrxCo2O5+delta system exhibits a structural change from orthorhombic (x = 0) to tetragonal (0.2 <= x <= 0.6) to orthorhombic (x = 1) with increasing Sr content. The difference in ionic radii between (Ba1-xSrx)(2+) and Gd3+ plays a dominant role in determining the oxygen-content value in GdBa1-xSrxCo2O5+delta, and the oxygen content and the oxidation state of cobalt increase with increasing Sr content. The electrical conductivity of the GdBa1-xSrxCo2O5+delta system increases with Sr content due to an increasing oxygen content and a straightening of the O-Co-O bonds as evidenced by the structural change from orthorhombic to tetragonal. Sr substitution also improves the chemical stability of the GdBa1-xSrxCo2O5+delta cathodes in contact with the Ce0.9Gd0.1O1.95 and La0.8Sr0.2Ga0.8Mg0.2O2.8 electrolytes at 1100 degrees C. The x = 0.2 and 0.6 samples with a tetragonal structure exhibit higher catalytic activity for ORR in single-cell solid oxide fuel cell than the x = 0 and 1.0 samples with an orthorhombic structure. (C) 2008 The Electrochemical Society.