Anodic dissolution of elemental Mg was examined in an ionic liquid, trimethyl-n-hexylammonium bis[(trifluoromethyl)sulfonyl]amide (TMHA-Tf2N), containing a simple salt, Mg(Tf2N)2, having a common anion. faradaic anodic dissolution was markedly stimulated in the presence of water dissolved within its solubility limit. The dissolution current efficiency for elemental Mg was found to be almost 100% without decreasing the water content, indicating that spontaneous reaction of Mg with water was limited and water molecules play a catalytic role in the overall dissolution mechanism, where a magnesium(II) oxide/hydroxide is involved. The Mg2+/Mg0 redox potentials, or onset potentials for Mg dissolution, vs. Li+/Li0 redox with and without water were also estimated from a set of sampled-current voltammograms obtained by a potential-step method. © 2013 The Electrochemical Society. [DOI: 10.1149/2.057310jes] All rights reserved.