The amorphous hydrous manganese oxide (denoted as a-MnO x ·nH2O) was anodically deposited from the MnSO4 solutions of various pH values. The capacitive characteristics and stability of this oxide without and with annealing in air for 2 h up to 400°C were systematically investigated in aqueous electrolytes through means of cyclic voltammetry (CV) and the constant-current charge–discharge method. The redox properties of a-MnO x ·nH2O were strongly affected by the electrolytes employed and this oxide exhibited ideally capacitive behavior in 0.1 M Na2SO4 and 0.3 M KCl. The stability of this amorphous hydrous oxide was enhanced by the annealing treatment while its capacitance was gradually decreased with increasing the annealing temperature. The amorphous structure and surface morphologies of a-MnO x ·nH2O with annealing at different temperatures were, respectively, examined in terms of the X-ray diffraction (XRD) patterns and scanning electron microscopic (SEM) photographs. The oxidation states of these a-MnO x ·nH2O deposits were studied by X-ray photoelectron spectroscopy (XPS).