Differential thermal analysis and X-ray diffarction have been applied to 6 birnessites from different localities in northeastern Japan. These thermal curves are characterized by a single flat endothermic peak at 600_??_750°C, due to the recrystallization of birnessite to hausmannite, but in some case this peak is absent, By heating, birnessite first inverts to cryptomelane then to amorphous state and lastly to hausmannite, but in some specimens no cryptomelane is formed.