The structural features of equilibrium phases, LiMn2O4 and Li2MnO3, and their acid-delithiated products (MnO2) are compared using x-ray diffraction and electron microscopy. The observed morphological changes associated with acid-delithiated Li0.1Mn2O4 confirm the acid digestion mechanism proposed by Hunter. Three delithiated phases including α-, γ-, and β-MnO2-like phases can be derived from Li2MnO3 under different acid digestion conditions. An acid-delithiation mechanism converting Li2MnO3 to α- and γ-MnO2 is proposed. This mechanism involves an acid-dissolution and precipitation process. A related scheme is also proposed for the formation of β-MnO2-like materials from α- and γ-MnO2 phases. The crystallography of the α-MnO2 structure is related to the grain/crystal geometry of α-MnO2 on the basis of convergent-beam electron diffraction and lattice imaging analysis. An in situ heating experiment in a transmission electron microscope demonstrates that heat-treatment of hydrated α-MnO2 at ∼300°C develops porosity within grains/crystals. It is proposed that the microstructural changes induced by acid delithiation may contribute to the reported difference in the initial electrochemical behavior of acid delithiated Li0.1Mn2O4 and equilibrium LiMn2O4. The delithiated product derived from Li2MnO3 at ∼115°C for 4-6 h, having γ-MnO2 as the major phase, demonstrates superior electrochemical properties.