X-ray diffraction (XRD), infrared (IR), and scanning electron microscopy (SEM) techniques have been used to monitor the crystallization of birnessite (Na4Mn14O27·9H2O, JCPDS card 32-1128) by aging MnO x , which was produced from the oxidation of Mn(OH)2 by KMnO4 in an NaOH solution in the presence of Mg2+. The crystallization process of birnessite can be divided into three stages:  an induction period, a fast crystallization period, and a steady-state period. The crystallization of birnessite was accompanied by the crystallization and phase transformation of feitknechtite (β-MnOOH, JCPDS card 18-0804). Increasing the temperature will reduce the induction period and accelerate the crystallization and phase transformation. Increasing the basicity has almost the same effect as increasing the aging temperature. The crystallization process is also influenced by varying the molar ratio of MnO4 -/Mn2+. A diagram of produced phases at room temperature at different basicities and ratios of MnO4 -/Mn2+ is obtained. The interconversion between birnessite and buserite (Na4Mn14O27·21H2O, JCPDS card 23-1046) is also discussed.