The alpha- and gamma-phases of MnO2 prepared by electrolysis of MnSO4 and KSO4 (where M = Li+, Na+, K+, Rb+, Cs+ or Mg2+) in aqueous solutions at various pH and voltage E, values under ambient conditions have been systematically studied. The structures of powdery MnO2 produced are found to depend on the radius of the W counter cation in addition to the pH and E-v conditions. In order to achieve the a-phase for MnO2 formation under neutral pH condition, the radius of counter cation must be equal to or greater than 1.41 angstrom, the size of the K+ cation. The relative concentration ratio of [MnO4-](transient)/[Mn (2+)], which is related to the pH-Ev conditions, also affects the structure of MnO2 produced with counter ions smaller than K+. For samples prepared in acidified solution with the counter ions of Li+, Na+ or Mg2+ at 2.2 V, the electrolysis products display the gamma-MnO2 phase while those prepared at 2.8 V electrolysis produce a mixture of gamma-MnO2 and alpha-MnO2 phases. Single phase of alpha-MnO2 is identified in the 5 V electrolysis products. Furthermore, the valence state of manganese was found to decrease as the applied voltage was reduced from 5.0 to 2.2 V. This implies that the lower [MnO4-](transient)[Mn2+] ratio or the less oxidative condition is responsible for the non-stoichiometric MnO2 structure with oxygen deficiency. (c) 2007 Elsevier Ltd. All rights reserved.