In this work, the electrochemical behavior and the corresponding mass variation of amorphous manganese oxide (denoted as a-MnO x ) are examined simultaneously in a neutral electrolyte containing 10mM Na2SO4 without or with NaHCO3 or Na2HPO4 by cyclic voltammetry with a quartz crystal microbalance (QCM). a-MnO x was anodically deposited onto the gold-sputtered quartz electrode by means of a potentiostatic method at 1.0V for 60s. The morphology and microstructure of a-MnO x with different potential windows of CV cycling in the above electrolyte were examined by means of the scanning electron microscopy (SEM). From this EQCM study, a-MnO x is unstable between 1.0 and 1.2V in 10mM Na2SO4 because of significant dissolution of a-MnO x . The repeated potential cycling promotes the electrocatalytic activity of a-MnO x for oxygen evolution, which also enhances the dissolution of a-MnO x . The dissolution phenomenon and oxygen evolution are successfully suppressed by the formation of insoluble manganese carbonate or the adsorption of phosphate by adding NaHCO3 or Na2HPO4 in the Na2SO4 electrolyte, enlarging the potential window for the charge/discharge of a-MnO x .