The Gibbs free energy of formation of CaRh2O4(s) has been determined using two techniques viz., quadrupole mass spectrometer coupled to a Knudsen cell and solid-state cell incorporating CaF2(s) as the solid electrolyte. In the former method, equilibrium O-2(g) pressures were measured over the phase field Rh(s) + Rh2O3(S), in the temperature range 793.7-909.1 K and over the three phase mixture CaRh2O4(S) + Rh(s) + CaO(s) was measured from 862.1 to 1022.7 K. The Gibbs free energy of formation of Rh2O3(S) from elements in their standard state can be given by Delta(f)Gdegrees(Rh2O3(S)) (KJ mol(-1) +/- 2.0) = -363.2 + 0.241T (K). The Gibbs free energy of formation of CaRh2O4(S) from elements in their standard state can be given by Delta(f)Gdegrees(CaRh2O4(S)) (kJ mol(-1) +/- 2.0) = -1030.5 + 0.3437T (K). In the electrochemical technique, the cell configuration employed was (-)Pt/O-2(g). {CaO(s) + CaF2(s)}//CaF2//{CaRh2O4(S) + Rh2O3(S) + CaF2(S)}, O-2(g)/Pt(+). The emf values were measured in the temperature range 879.7-1000K can be represented by the following expression: E(V)(+/-7.63 x 10(-4)) = 0.3928 - 2.374 x 10(-4)T (K). From the measured emf of the cell and requisite Delta(f)Gdegrees values from the literature, Delta(f)Gdegrees(CaRh2O4(S)) from elements in their standard state has been calculated and can be represented by Delta(f)Gdegrees(CaRh2O4(S)) (kJ mol(-1) +/- 2.0) = -1079 + 0.390T (K). The uncertainty estimates for DeltafGdegrees include the standard deviation in the emf and uncertainty in the data taken from the literature. The slope and intercept of the above equation gives the entropy and enthalpy of formation of the compound at the average experimental temperature T-av = 940 K. (C) 2004 Elsevier B.V. All rights reserved.