An x‐ray diffraction study has been made of the various types of manganese dioxide with particular emphasis on those prepared chemically. A review of the literature suggests that several types have received different names by different investigators. A diffraction pattern of ramsdellite, uncontaminated by the lines of pyrolusite, is reported. Three modifications of
γ
‐
MnO
2
are described and their relation to ramsdellite and pyrolusite is discussed. The manner in which lead is combined in a commercial manganese dioxide by‐product, and the part played by potassium in determining the course of various transformations, are also discussed.