Vapor pressure of a mixture of sodium carbonate Na 2 CO 3 and ferric oxide Fe 2 O 3 (molar ratio 2:3) has been measured with a Knudsen effusion mass spectrometer at temperatures in the range (918 to 1023) K. Sodium ferrite Na 4 Fe 6 O 11 was identified as the solid while CO 2 (g) was the vapor species. From the temperature dependence of p(CO 2 ), thermodynamic data for Na 4 Fe 6 O 11 was evaluated assuming the reaction 2Na 2 CO 3 +3Fe 2 O 3 =Na 4 Fe 6 O 11 +2CO 2 . The standard molar enthalpy of formation of Na 4 Fe 6 O 11 at T=298.15 K was determined to be, ΔfHm∘(Na4Fe6O11,298.15K)=-(3569.54±3.95)kJ·mol-1. The standard molar Gibbs function of formation at T=298.15 K was evaluated as ΔfGm∘(Na4Fe6O11,298.15K)=-(3255.3±12.4)kJ·mol-1