Vapor pressure of a mixture of sodium carbonate Na2CO3 and ferric oxide Fe2O3 (molar ratio 2:3) has been measured with a Knudsen effusion mass spectrometer at temperatures in the range (918 to 1023) K. Sodium ferrite Na4Fe6O11 was identified as the solid while CO2 (g) was the vapor species. From the temperature dependence of p(CO2), thermodynamic data for Na4Fe6O11 was evaluated assuming the reaction 2Na(2)CO(3) + 3Fe(2)O(3) = Na4Fe6O11 + 2CO(2). The standard molar enthalpy of formation of Na4Fe6O11 at T = 298.15 K was determined to be, Delta(f)H(m)(o)(Na4Fe6O11, 298.15 K) = -(3569.54 +/- 3.95) U (.) mol(-1). The standard molar Gibbs function of formation at T = 298.15 K was evaluated as Delta(f)G(m)(o)(Na4Fe6O11,298.15 K) = -(3255.3 +/-12.4) kJ (.) mol(-1). (c) 2004 Published by Elsevier Ltd.