The Gibbs free energy of formation of Nd 3RuO 7(s) has been determined using solid-state electrochemical cell employing oxide ion conducting electrolyte. The electromotive force (e.m.f.) of the following solid-state electrochemical cell has been measured, in the temperature range from 929.3 to 1228.6 K. Cell: (−)Pt/{Nd 3RuO 7(s) + Nd 2O 3(s) + Ru(s)}//CSZ//O 2( p(O 2) = 21.21 kPa)/Pt(+) The Gibbs free energy of formation of Nd 3RuO 7(s) from elements in their standard state, calculated by the least squares regression analysis of the data obtained in the present study, can be given by: {Δ f G°(Nd 3RuO 7, s)/(kJ mol −1) ± 1.6} = −3074.3 + 0.6097( T/K); (929.3 ≤ T/K ≤ 1228.6). The uncertainty estimate for Δ f G°( T) includes the standard deviation in e.m.f. and the uncertainty in the data taken from the literature. The intercept and the slope of the above equation correspond to the enthalpy of formation and entropy, respectively, at the average experimental temperature of T av. = 1079 K.