Amorphous manganese oxide deposits with nanostructures (denoted as a-MnOx . nH(2)O) were electrochemically deposited onto graphite substrates from 0.16 M MnSO4 . 5H(2)O with pH 5.6 by means of the potentiostatic, galvanostatic, and potentiodynamic techniques. The maximum specific capacitance of a-MnOx . nH(2)O deposits plated in different modes, measured from cyclic voltammetry at 25 mV s(-1), is about 230 F g(-1) in a potential window of 1.0 V. The high electrochemical reversibility, high-power characteristics, good stability, and improved frequency responses in 0.1 M Na2SO4 for these nanostructured a-MnOx . nH(2)O deposits prepared by electrochemical methods demonstrate their promising potential in the application to electrochemical super-capacitors. The nanostructure of a-MnOx . nH(2)O, clearly observed by means of a scanning electron microscope, was found to depend strongly on the deposition mode. The similar capacitive performance of all deposits prepared in different modes was attributable to their nonstoichiometric nature with a very similar oxidation state, demonstrated by XPS spectra. (C) 2003 The Electrochemical Society.