Standard molar Gibbs free energies of formation from the oxides, Delta(f)G(ox)degrees, of Ca2ZrSi4 O-12 and Ca3ZrSi2O9 have been determined in the temperature range 923 K to 1273 K with the help of solid-state galvanic cells involving calcium beta" -alumina electrolyte. The Delta(f)G(ox)degrees values obtained for Ca2ZrSi4O12, have been compared with those by Jacob et al. (c) 2004 Elsevier B.V. All rights reserved.