The effects of NH4+ and Cl- ions on the corrosion of 63-37 brass have been investigated using the electrochemical impedance spectroscopy (EIS) technique. For this purpose, 10(-3) 10(-2), and 10(-1)M sodium chloride, sodium sulphate, and ammonium sulphate solutions were used. Impedance data have been obtained at the corrosion potential with a two electrode system. The impedance diagrams (Nyquist plots) were evaluated according to the semi-ellipse model. A new equivalent circuit model has been suggested, and a differential capacitance definition has been created within this equivalent circuit model. The polarisation resistance has been divided into the diffuse layer resistance (R-d) and the charge transfer resistance (R-t). The R-t values, which have been calculated from impedance diagrams for all solutions, indicated that the ammonium ions ave corrosive to 63-37 brass, the corrosive effect depending on the ammonium concentration. However, chloride ions did not show corrosive effects at the corrosion potential.