Summary: X-ray and thermal studies in the system MnO-O-H₂O show that the transformations groutite(α-MnOOH)-ramsdellite(MnO₂)-pyrolusite(MnO₂) are topotactic, with the a, b, and c directions of all three corresponding. The reactions proceed by migration of manganese atoms and, where appropriate, protons. Pure single crystals of ramsdellite could not be prepared by heating groutite, even though the two structures are very similar.