The effect of relative humidity on the atmospheric corrosion of defective polyester-coated galvanized mild steel was investigated using electrochemical impedance spectroscopy. Measurements were carried out on samples which had been tested in an accelerated atmospheric corrosion test. The relative humidity was adjusted to the desired value (70, 80 and 100%) by means of water-glycerol mixture in 0.1 M ammonium sulphate solutions. The impedance diagrams (Nyquist plots) were simulated using Equivert program developed by Boukamp using a recognized model electrical analogue and semi-ellipse model developed by Erbil. It has been shown that the defective coating resistance and corrosion rate of metal under the coating changes with relative humidity. A new equivalent circuit model has been suggested and also a differential capacitance definition has been created in this equivalent circuit model and charge transfer resistance (R-ct) is redefined in this study. (C) 2000 Elsevier Science Ltd. All rights reserved.