Kinetic and mechanistic features have been studied for the crystallization of birnessite in aqueous systems via different synthesis methods: the oxidation of Mn2+, reduction of MnO4-, and odor reaction between Mn2+ and MnO4-. For oxidation methods, a topotactical conversion from Mn(OH)(2) to birnessite via feitknechtite (beta-MnOOH) is observed. In reduction methods, birnessite evolves from the initially produced amorphous manganese oxide (AMO gel). For redox methods, both mechanisms exist, with the latter prevailing. A liquid mechanism is proposed to describe the reduction and redox synthesis, which comprises three stages: an induction period, a fast crystallization period, and a steady-state period. The redox method is accompanied by the formation and phase transformation of feitknechtite to birnessite. A method combining LR and XRD quantitation is proposed to detect nuclei in the induction period. Crystallization rates and apparent energies of activation of crystallization for reduction and redox methods are determined.