Standard enthalpies of formation of GdBaCo2-xMxO5.5 (M = Fe, Mn; x=0, 0.2) from constituent oxides. Delta H-ox(i)o, and elements, Delta H-f(i)o, at 298 K were calculated using dissolution enthalpies of corresponding double perovskites. Both Delta H-ox(i)o and Delta H-f(i)o were found to decrease in the sequence of GdBaCo2O5.5, GdBaCo1.8Fe0.2O5.5 and GdBaCo1.8Mn0.2O5.5 indicating growth of relative thermodynamic stability of GdBaCo2O5.5 caused by the substitution of Fe or Mn for Co. (C) 2011 Elsevier B.V. All rights reserved.