The effect of the microstructure on the electrochemical passivation and the corrosion of an Al-Zn-Mg alloy (5.00% Zn, 1.66% Mg, 0.23% Cu and 0.053% Nb) in 0.1 mol dm(-3) NaCl, has been studied using ocp measurements, cv and EIS. A variety of microstructures, in which the basic differences were the aluminium matrix composition and the size of the MgZn2 precipitates, was obtained using well known heat treatments. The corrosion potentials in oxygen-saturated (1 atm) 0.1 mol dm(-3) NaCl and the repassivation potentials in 0.1 mol dm(-3) NaCl were determined, increasing in the order F < A approximate to ST < B approximate to C. The impedance measurements were performed at constant potential after achieving constant current conditions. Impedance spectra for all the specimens examined within the passive potential region were semicircular when data was presented in the Nyquist format. However, in the pitting potential region, inductive semicircles appeared. These were assigned to the relaxation of adsorbed species. The depression of the capacitive semicircles was correlated with the size of the MgZn2 precipitates. Equivalent circuits that simulate the experimental impedance diagrams were developed on the basis of the microstructural observations.