An electrochemical quartz crystal microbalance (EQCM) system combined with a wall jet disk electrode (WJDE) was applied to the investigation of copper dissolution in acidic chloride media. The average valence of dissolved ions, It, was calculated from the resonant frequency of the quartz crystal and the applied current. The value of n between 1 and 2 indicates that both Cu(I) and Cu(II) dissolved from the copper electrode at the same time. The formation rate of Cu(II) increased with decreasing chloride concentration. The influence of the hydrodynamic conditions on the dissolution behaviour was discussed.