Standard molar Gibbs energies of formation of UTeO5(s) have been determined by measuring the relative oxygen chemical potential over the three phase system {U3O8(s) +TeO2(s) +UTeO5(s)} in the temperature range (821 to 993.5) K using a solid electrolyte galvanic cell having a 0.15 mole fraction calcia-stabilized zirconia (CSZ) as an electrolyte. The cell used was: Pt \ {U3O8(s) + TeO2(s) + UTeO5(s)} parallel to CSZ parallel to {Ni(s) + NiO(s)} \ Pt. The observed e.m.f. values are represented by: (E+/-1 . 10(-3))/V = 0.7677-6.290 . 10(-4) (T/K). The relative oxygen chemical potential over the three phase system was evaluated from the e.m.f. values and is given as: {Delta mu(O-2) +/- 1.4}/(kJ . mol(-1)) = -759.8+0.4222 (T/K). The standard molar Gibbs energies of formation of UTeO5(s) are evaluated from the oxygen chemical potential data, and the Delta(f)G(m)(o)(T) of U3O8(s) and TeO2(s) from literature. The resulting standard molar Gibbs energies of formation of UTeO5(s) are given by: {Delta(f)G(m)(o)(UTeO5, s, T) +/- 4.6}/(kJ mol(-1)) = -1642.5 + 0.4784 (T/K). Using the literature values, the Delta(f)H(m)(o)(UTeO5, s, 298.15) has been calculated by using the second- and third-law methods. The resulting values are {-(1638.8 +/- 9) and -(1601.6+/-10)} kJ.mol(-1), respectively.