Pseudocapacitive charge-storage reaction of MnO 2 •nH 2 O in several aqueous alkali and alkaline salts solutions, including LiCl, NaCl, KCl, CsCl, and CaCl 2 , has been studied on fine-grained MnO 2 •nH 2 O thin films and particles which possess the-MnO 2-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 H 3 O + plays the predominant ͑Ͼ60%͒ 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: Mn͑IV͒O 2 •nH 2 O + ␦e − + ␦͑1 − f͒H 3 O + + ␦fM + ͑H 3 O͒ ␦͑1−f͒ M ␦f ͓Mn͑III͒ ␦ Mn͑IV͒ 1−␦ ͔O 2 •nH 2 O, where M + is alkali cation.