Standard enthalpies of formation of double perovskites REBaCo2O6-delta (RE=Gd, Pr) from elements, Delta H-f(i)degrees, at 298 K were determined as a function of their oxygen content using isothermal solution calorimetry. Delta H-f(GdBaCo2O6-delta)degrees and Delta H-f(PrBaCo2O6-delta)degrees were found to increase linearly with oxygen nonstoichiometry. Such behavior indicates in favor of decrease of relative thermodynamic stability of the double perovskites with oxygen nonstoichiometry. (C) 2014 Elsevier B.V. All rights reserved.