Standard enthalpies of formation of GdBaCo2−xMxO5.5 (M=Fe, Mn; x=0, 0.2) from constituent oxides, ΔoxHi∘, and elements, ΔfHi∘, at 298K were calculated using dissolution enthalpies of corresponding double perovskites. Both ΔoxHi∘ and ΔfHi∘ 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.