The rates of hydrogen evolution, magnesium (Mg) dissolution and electron exchange were monitored individually, in situ, and in real time during anodic and cathodic polarization of Mg in 0.1 M NaCl. Under all conditions, the anodic charge correlated with Mg2+ dissolution while hydrogen formation was independent and led to insoluble Mg(II) formation. Increased iron (Fe) impurity promoted hydrogen evolution, but was not necessary for the negative difference effect. These results suggest that electrochemical dissolution occurred across an intact Mg(II)-based film, while hydrogen evolution occurred in regions where the film breaks down induced by anodic polarization.