The chemical potentials of CaO in two-phase fields (TiO2 + CaTiO3), (CaTiO3 + Ca4Ti3O10), and (Ca4Ti3O10 + Ca3Ti2O7) of the pseudo-binary system (CaO + TiO2) have been measured in the temperature range (900 to 1250) K, relative to pure CaO as the reference state, using solid-state galvanic cells incorporating single crystal CaF2 as the solid electrolyte. The cells were operated under pure oxygen at ambient pressure. The standard Gibbs free energies of formation of calcium titanates, CaTiO3, Ca4Ti3O10, and Ca3Ti2O7, from their component binary oxides were derived from the reversible e.m.f.s. The results can be summarised by the following equations: CaO(solid) + TiO2(solid) -> CaTiO3(solid), Delta G degrees +/- 85/(J.mol(-1)) = - 80,140 -6.302(T/K); 4CaO(solid) + 3TiO(2)(solid) -> Ca4Ti3O10(solid), Delta G degrees +/- 275/(J.mol(-1)) = - 243,473 - 25.758(T/K); 3CaO(solid) + 2TiO(2)(solid) -> Ca3Ti2O7(solid), Delta G degrees +/- 185/(J.mol(-1)) = - 164,217 - 16.838(T/K). The reference state for solid TiO2 is the rutile form. The results of this study are in good agreement with thermodynamic data for CaTiO3 reported in the literature. For Ca4Ti3O10 Gibbs free energy of formation obtained in this study differs significantly from that reported by Taylor and Schmalzried at T = 873 K. For Ca3Ti2O7 experimental measurements are not available in the literature for direct comparison with the results obtained in this study. Nevertheless, the standard entropy for Ca3Ti2O7 at T = 298.15 K estimated from the results of this study using the Neumann-Koop rule is in fair agreement with the value obtained from low-temperature heat capacity measurements. (C) 2009 Elsevier Ltd. All rights reserved.