The Gibbs energy of formation of Ba(2.87)sUO(5.875)(s) has been determined using manometric technique. The equilibrium CO2(g) pressures over the three phase mixture: {Ba2.875UO5.875(s) + Ba3UO6(s) + BaCO3(s)} have been measured using an absolute pressure transducer in the temperature range 1030-1220 K. The corresponding Gibbs energy expression as a function of temperature can be given as: Delta(f)G degrees(m)(T)(Ba2.875UO5.875, s,T) (kJ mol(-1)) +/- 5 = -3153.1 + 0.5573 (T/K) Delta H-f degrees(m)(298.15 K) and S degrees(m)(298.15 K) data have been calculated from the above Gibbs energy expression using required values from the literature. The corresponding values are -3143.4 +/- 2 kJ mol(-1) and 270.8 +/- 4J K-1 mol(-1), respectively. The Delta H-f degrees(m)(298.15 K) value has also been calculated from third law method as -3137.7 +/- 4 kJ mol(-1). (c) 2010 Elsevier B.V. All rights reserved.