Pseudocapacitive charge-storage reaction of MnO(2)(.)nH(2)O in several aqueous alkali and alkaline salts solutions, including LiCl, NaCl, KCl, CsCl, and CaCl2, has been studied on fine-grained MnO(2)(.)nH(2)O thin films and particles which possess the epsilon-MnO2-type crystal structure. In situ synchrotron X-ray diffraction analysis shows that charge transfer at Mn sites upon reduction/oxidation of MnO(2.)nH(2)O is balanced by bulk insertion/extraction of the solution cations into/from the oxide structure, which causes reversible expansion and shrinkage in lattice spacing of the oxide during charge/discharge cycles. Electrochemical quartz-crystal microbalance and X-ray photoelectron spectroscopy data further indicate that H3O+ plays the predominant (> 60%) role in all cases, while the extent of participation of alkali cations first decreases and then increases with ionic size. The charge-storage reaction can be summarized as: Mn (IV)O(2)(.)nH(2)O + delta e(-) + delta(1 - f ) H3O+ + delta fM(+) reversible arrow (H3O)(delta(1-f))M-delta f [Mn (III)(delta)Mn(IV)(1-delta)]O(2)(.)nH(2)O, where M+ is alkali cation.