A study of the manganese dioxides and other crystalline materials encountered in a dry cell has been made and five different crystal varieties of identified as follows : (a) pyrolusite, (b) ramsdellite, (c) cryptomelane, (d) γ and (e) δ . The last two forms are poorly crystallized. Examination of the oxides by means of the electron microscope revealed great differences in particle size and shape. Prismatic crystals of were found in the pastes of undischarged cells, while occurred in small hexagonal plates in the pastes of discharged cells. X‐ray diffraction studies of thoroughly discharged mixes indicate that the manganese dioxide of a dry cell is reduced by discharge to the double salt, (heterolite).