!The standard molar Gibbs free energies of formation of TbFeO3(s) and Tb3Fe5O12(s) have been determined using solid-state electrochemical cell employing different solid electrolytes. The reversible emfs of the following solid-state electrochemical cells have been measured in the temperature range 1050 less than or equal to T/K less than or equal to 1250.Cell (I) : (-)Pt/{TbFeO3(s) + Tb2O3(s) + Fe(s)}//YDT/CSZ//{ Fe(s)+ Fe0.95O(s)}/Pt(+)Cell (II) : (-)Pt/{Fe(s) + Fe0.95O(s)}//CSZ//{TbFeO3(s)+ Tb3Fe5O12(s) + Fe3O4(s)}/Pt(+)The oxygen chemical potentials corresponding to the three-phase equilibria involving the ternary oxides have been computed from the emf data. The standard molar Gibbs free energies of formation of solid TbFeO3 and Tb3Fe5O12 calculated by the least-squares regression analysis of the data obtained in the present study are given by{Delta(f)G(m)(o)(TbFeO3, s)/(kJ(.)mol(-1)) +/- 3.2} = -1357.5 + 0.2531 (.) (T/K); (1050 less than or equal to T/K less than or equal to 1548);and{Delta(f)G(m)(o)(Tb3Fe5O12,s)/(kJ(.)mol(-1)) +/- 3.5} = -4901.7 + 0.9997. (T/K); (1050 less than or equal to T/K less than or equal to 1250).The uncertainty estimates for Delta(f)G(m)(o). include the standard deviation in the emf and uncertainty in the data taken from the literature. Based on the thermodynamic information, oxygen potential diagram and chemical potential diagrams were computed for the system Tb-Fe-O at T = 1250 K. (C) 2003 Elsevier Ltd. All rights reserved.