The standard molar Gibbs energies of formation of LnFeO₃(s) and Ln₃Fe₅O₁2(s) where Ln=Eu and Gd have been determined using solid-state electrochemical technique employing different solid electrolytes. The reversible e.m.f.s of the following solid-state electrochemical cells have been measured in the temperature range from 1050 to 1255K.Cell (I): (−)Pt / {LnFeO₃(s)+Ln₂O₃(s)+Fe(s)} // YDT/CSZ // {Fe(s)+Fe_0.95O(s)} / Pt(+);Cell (II): (−)Pt/{Fe(s)+Fe_0.95O(s)}//CSZ//{LnFeO₃(s)+Ln₃Fe₅O₁₂(s)+Fe₃O₄(s)}/Pt(+);Cell (III): (−)Pt/{LnFeO₃(s)+Ln₃Fe₅O₁₂(s)+Fe₃O₄(s)}//YSZ//{Ni(s)+NiO(s)}/Pt(+);and Cell(IV):(−)Pt/{Fe(s)+Fe_0.95O(s)}//YDT/CSZ//{LnFeO₃(s)+Ln₃Fe₅O₁₂(s)+Fe₃O₄(s)}/Pt(+).The oxygen chemical potentials corresponding to the three-phase equilibria involving the ternary oxides have been computed from the e.m.f. data. The standard Gibbs energies of formation of solid EuFeO₃, Eu₃Fe₅O₁₂, GdFeO₃ and Gd₃Fe₅O₁₂ calculated by the least-squares regression analysis of the data obtained in the present study are given byΔfG°m(EuFeO₃, s) /kJmol⁻¹ (±3.2)=−1265.5+0.2687(T/K) (1050 ⩽ T/K ⩽ 1570),ΔfG^°_m(Eu₃Fe₅O₁₂, s)/kJ.mol⁻¹ (± 3.5)=−4626.2+1.0474(T/K) (1050 ⩽ T/K ⩽ 1255),ΔfG^°_m(GdFeO₃, s) /kJmol⁻¹ (± 3.2)=−1342.5+0.2539(T/K) (1050 ⩽ T/K ⩽ 1570),and ΔfG^°_m(Gd₃Fe₅O₁₂, s)/kJ.mol⁻¹ (± 3.5)=−4856.0+1.0021(T/K) (1050 ⩽ T/K ⩽ 1255).The uncertainty estimates for ΔfG^°_m include the standard deviation in the e.m.f. and uncertainty in the data taken from the literature. Based on the thermodynamic information, oxygen potential diagrams for the systems Eu–Fe–O and Gd–Fe–O and chemical potential diagrams for the system Gd–Fe–O were computed at 1250K.