An isothermal section of the phase diagram of the system Co-Sb-O at 873 K was established by isothermal equilibration and XRD analyses of quenched samples. The following galvanic cells were designed to measure the Gibbs energies of formation of the three ternary oxides namely CoSb2O4, Co7Sb2O12 and CoSb2O6 present in the system. Chromel, Mo, Sb, CoO, CoSb2O4 15 CSZ NiO, Ni, Mo, Chromel I Pt, CoO, Co7Sb2O12,CoSb2O4 15 CSZ air (P-o2 = 0.21 atm), Pt II Pt, Co7Sb2O12, CoSb2O6, CoSb2O4 15 CSZ air (P (o2) = 0.21 atm), Pt III where 15 CSZ stands for ZrO2 stabilized by 15 mol % CaO. The reversible emfs obtained could be represented by the following expressions. E (I) +/- 0.5 (mV) = 120.7 - 0.04924 T (K) (795-954 K) E (II) +/- 0.5 (mV) = 1106.3 - 0.3992 T (K) (790-1040 K) E (III) +/- 0.4 (mV) = 967.9 - 0.4395 T (K) (855-1035 K) The standard Gibbs energies of formation of CoSb2O4, Co7Sb2O12 and CoSb2O6 were computed from the emf expressions: DeltaG(f)(o) (CoSb2O4) +/- 4.3 (kJ mol(-1)) = -1006.3 + 0.3549 T (K) DeltaG(f)(o) (Co7Sb2O12) +/- 10.5 (kJ mol(-1)) = -2834.5 + 0.9190 T (K) DeltaG(f)(o) (CoSb2O6) +/- 4.5 (kJ mol(-1)) = -1379.9 + 0.5115 T (K) The reasonability of the above data were assessed by computing the entropy change for the solid-solid reactions leading to the formation of ternary oxides from the respective pairs of constituent binary oxides.