Among the perovskites, the rare earth manganites find application in several electrochemical devices because of their enhanced thermodynamic stability. in this paper, we present the results obtained on the preparation and characterization of La0.95MnO3+delta and Sm0.95MnO3+delta which were prepared by the solid state and sol-gel methods. XRD characterization of the manganites indicated that the crystal structure depends on the method of preparation and heat treatments. The ratio of Mn3+ to Mn4+ in these samples also depended on the method of preparation and heat treatments, as indicated by thermogravimetric(TG) and temperature programmed reduction (TPR) studies in Ar+5% H-2 atmosphere. The standard molar enthalpy of formation, which is a measure of the thermodynamic stability of these compounds were determined using an isoperibol calorimeter. (C) 2009 Elsevier B.V. All rights reserved.