The soluble and solid-state intermediates formed during redox cycling of electrodeposited manganese dioxide (EMD), birnessite and chemically modified EMD (Bi-CMEMD), and birnessite (Bi-birnessite) electrodes were investigated using a stationary detector electrode (soluble intermediates) and x-ray diffraction (solid-state intermediates). Reduction of each electrode type can be divided into a homogeneous stage followed by a heterogeneous stage. For all electrode types, homogeneous reduction was a solid-state process involving proton and electron insertion into the manganese dioxide structure, causing a lattice expansion. Toward the end of homogeneous EMD reduction, soluble species were detected, presumably due to an equilibrium shift between solid and solution phase Mn3+ species. The homogeneous/heterogeneous transition was also electrode dependent; i.e., similar to MnO1.55 for EMD and Bi-CMEMD, similar to MnO1.83 for birnessite, and similar to MnO1.73 for Bi-birnessite. Heterogeneous electrochemical behavior was also electrode dependent. Initial heterogeneous reduction of EMD, Bi-birnessite, and Bi-CMEMD proceeded through a soluble Mn3+ intermediate to form Mn(OH)(2). Electrolyte concentration effects were more pronounced in this stage, since more concentrated KOH electrolytes lead to greater Mn3+ solubility. The composition at which Mn(OH)(2) was first detected in the Bi-birnessite electrode suggested that the Mn(IV) to Mn(III) and Mn(III) to Mn(II) reduction processes occurred simultaneously. Heterogeneous reduction of birnessite was a solid-state process that resulted in Mn3O4, which is electrochemically inactive. Mn(OH)(2) oxidation resulted in formation of birnessite, the exact nature of which depended on the presence or absence of Bi3+ ions. Under these deep discharge cycling conditions, the EMD electrode behaved poorly due to the eventual formation of Mn3O4. However, the Bi-birnessite and Bi-CMEMD electrodes are rechargeable due to the presence of Bi3+ ions, which prevent Mn3O4 formation.