The susceptibility to the electrochemical corrosion of homogeneous Al-Mg alloys containing 0.33%, 0.43%, 0.77% and 0.88% (in weight) Mg in NaCl solutions has been studied by means of ocp measurements, cv, EIS, optical microscopy, SEM and EDX. The breakdown and repassivation potentials of these alloys were shifted in the negative direction when the Mg content was increased, indicating an increase in their susceptibility to localized corrosion in the same direction. The impedance measurements were interpreted according to suitable equivalent circuits. In the passive region, two capacitive semicircles were obtained, the first being related with the film capacitance and the second one with a diffusion process in the oxide film itself. The oxide film capacitances on the Al-Mg alloys were greater than those found on high purity Al, suggesting the formation of more defective and hydrated films on these alloys. Pit nucleation of Al-Mg alloys is discussed considering the formation of a defective oxide. In the pitting potential region, the Nyquist diagrams consisted of a capacitive semicircle followed by one or two inductive semicircles. The film capacitances were greater than those found for the passive oxide films because of the chloride-rich film formation in pitting conditions. In addition, the film capacitances increased with Mg content of the alloy because of the increase in the pitted surface area,confirmed by SEM, in the same direction.