The corrosion behavior of a pure aluminum/pure magnesium couple in a weakly conductive sodium sulfate solution was investigated. Potential and current distributions on the surface of the model couple at the beginning of immersion were obtained by solving the Laplace equation using a finite element method (FEM) algorithm. It was shown that magnesium was the anode of the system while both oxygen and water reduced on aluminum. Calculations predicted a large current peak at the Al/Mg interface related to a local increase of both Mg dissolution and oxygen and water reduction on aluminum. This led to a local pH increase. Optical and scanning electron microscope observations confirmed a strong dissolution of magnesium combined with depassivation of aluminum at the Al/Mg interface. Local electrochemical impedance spectroscopy showed the detrimental effect of the galvanic coupling both on aluminum and magnesium.