Birnessite occurs in a wide variety of natural environments, and plays an important role in soil chemistry.A modified Stahli procedure was used to synthesize sodium birnessite in an alkali medium by O2 oxidation.The effects of preparative parameters on the synthesis of birnessite, such as pretreatment on solutions with N2,reaction temperature, O2 flow rate, fluxion velocity of the reaction suspension, and dehydration conditions were investigated. The fluxion velocity of the reactive suspension and O2 flow rate significantly influenced the synthesis of birnessite. Vigorous stirring raised the fluxion velocity of the reaction suspension and easily allowed synthesis of pure crystalline birnessite. However pretreatment of the reacting solutions with N2 and the reaction temperature had little effect on the synthesis. Diffusion of O2 was the controlling step during the course of oxidation. The optimum synthetic conditions for pure birnessite were: a NaOH to Mn molar ratio of 13.7, an O2 flow rate of 2 L min-1, and oxidation for 5 hours with vigorous stirring at normal temperatures. The chemical composition of the synthesized pure birnessite was Nao.25MnO2.07·0.66H2O.