Influence of gas-solid equilibrium and partial pressures of gaseous species was investigated on solid state reactions of Na2CO 3 and Co3O4 for synthesis of bulk sintered bodies of layered thermoelectric oxide NaCo2O4. It was revealed that the presence of CO2 in the reaction atmosphere strongly inhibits the formation reaction of NaCo2O4. However, precise control of the CO2 partial pressure in the oxygen atmosphere enabled us to avoid otherwise significant Na loss during sintering at temperature as high as 900°C, thereby allowing high reaction temperature required for incorporation of Na into the crystal lattice. Selective quantification of the Na species in order to distinguish those within the crystallites and at the grain boundaries revealed that the samples synthesized in the presence of dilute CO2 contain more Na in the crystallites with less Na segregation at the boundaries. Improved interconnection between the grains containing larger amounts of Na was suggested as a reason of higher electrical conductivity and larger Seebeck coefficient of the samples thus obtained, compared to the samples containing the same amount of total Na but synthesized in air.