The electrochemical behavior of magnesium perchlorate in an ionic liquid, 1,2-dimethyl-3-octylimidazolium bis(trifluoromethanesulfonyl)imide, is characterized using RuO2 as the cathode and Mg foil as the anode material. Electrochemical results indicated that the maximum Mg content intercalated into RuO2 resulted in Mg0.25RuO2, with an initial capacity of 101 mAh/g. X-ray diffraction studies of the Mg-intercalated RuO2 indicated a large expansion of the unit cell and a decrease from cubic to monoclinic symmetry. X-ray diffraction studies and structural analysis suggest a similarity between the structure of Mg0.25RuO2 and Li2RuO3. Charge/discharge experiments at a C/3 rate were performed vs. Mg metal. Results indicated that the capacity decreased significantly with cycling, most likely due to the strong interaction of Mg2+ with the oxide lattice of RuO2. Published by Elsevier Ltd.