The free energy, enthalpy, and entropy changes of the reaction 2In(I)+ 3NiO(s)= In2O3(s)+ 3Ni(s) have been derived from electromotive force measurements on the galvanic cell Ni(s), NiO(s)|(0·85 ZrO2+ 0·15CaO)| In2O3(s), In(I) In the temperature range 450–700° the free energy change of the above reaction was found to be ΔG°(kcal. mole–1)=–(52·33 ± 0·25)+(16·18 ± 0·4)·10–3T which, together with the free energy of formation of nickel oxide, gives the following equation for the free energy of formation of indium oxide: ΔG°(kcal. mole–1)=–(222·7 ± 1·0)+(78·1 ± 0·7)·10–3T The enthalpy of formation and the entropy of indium oxide at 298°K calculated from the results are –221·2 ± 1·1 kcal. mole–1 and 25·8 ± 0·8 e.u.