Magnesium anodization was examined in room temperature AlCl3:EMIC and AlCl3:DMPIC ionic liquids, where EMIC = 1-ethyl-3-methylimidazolium chloride and DMPIC = 1, 2-dimethyl-3-propylimidazolium chloride. For all melts, the AlCl3:organic chloride mole ratio was <1, yielding chloride-rich (i.e., basic) compositions. The rate of magnesium anodization was limited by diffusion of chloride ions to the electrode surface. From the Cottrell slopes for magnesium anodization at a Mg disk electrode and for chloride oxidation at a Pt disk electrode, the chloride stoichiometry of the anodization process in AlCl3:EMIC was determined to be 4.1 (+/- 0.5), corresponding to the formation of soluble MgCl42-. Similar chloride stoichiometry was found in AlCl3:DMPIC. MgCl2 buffers the melt to approximate neutrality from the basic side, dissolving as MgCl42-. Magnesium metal was chemically stable in basic AlCl3:DMPIC, but it reacted completely and irreversibly with basic AlCl3:EMIC to produce colored organic byproducts. Some comments are made on the acidity of NiCl2 and CdCl2 in the basic melts.