Several kinds of MnO2 were progressively reduced by cinnamic alcohol (CA) and aqueous hydrazine solutions (AHS) to compare changes in their structure. With alpha-MnO2 stabilized by NH4+, the maximum homogeneous degree of H-insertion (MHID) is only 0.62 H per Mn, which involves the filling of each NH4+-free tunnel by four protons. This MHID value is consistent with the discharge capacity during the electrochemical reduction in 1 M KOH solution and in nonaqueous media (similar to 0.65 and similar to 0.63 faradays per Mn, respectively). This result shows that Li+ and H+ ions occupy the same sites. The lowest degrees of oxidation are obtained when AHS are : used, resulting in progressive appearance of a spinel structure which replaces the original lattice. For degrees of reduction x lower than MnO1.33, pyrochroite exists in a poorly crystallized form since it is not observed in the XRD patterns. The XRD patterns of gamma-MnO2 reduced to MnO1.12 usually show the spinel structure while the patterns of the Bi-doped MnO2 reduced to MnO1.14 exhibit peaks corresponding to pyrochroite and bismite (Bi2O3) Thus, the presence of Bi3+ hinders the formation of the nonelectroactive compound Mn3O4 or gamma-Mn2O3, but the mechanism to explain this cannot be determined by XRD data alone.