Two compounds, BaNd2Fe2O7(s) and BaNdFeO4(S) in the quaternary system Ba-Nd-Fe-O were prepared by citrate-nitrate gel combustion route and characterized by X-ray diffraction analysis. Heat capacities of these two oxides were measured in two different temperature ranges: (i) 130-325 K and (ii) 310-845 K, using a heat flux type differential scanning calorimeter. Two different types of solid-state electrochemical cells with CaF2(s) as the solid electrolyte were employed to measure the e.m.f. as a function of temperature. The standard molar Gibbs energies of formation of these quaternary oxides were calculated as a function of temperature from the e.m.f. data. The standard molar enthalpies of formation from elements at 298.15 K, Delta H-f(m)degrees (298.15 K) and the standard entropies, S-m degrees (298.15 K) of these oxides were calculated by the second law method. The values of Delta H-f(m)degrees (298.15 K) and S-m degrees (298.15 K) obtained for BaNd2Fe2O7(s) are: -2756.9 kJ mol(-1) and 234.0 J K-1 mol(-1) whereas those for BaNdFeO4(s) are: -2061.5 kJ mol(-1) and 91.6 J K-1 mol(-1), respectively. (c) 2006 Elsevier B.V. All rights reserved.