Electrochemical quartz crystal microbalance (EQCM), which can detect surface mass change in ng scale, was employed to investigate the deposition of Li-O-2 species on a gold electrode upon reduction of oxygen in a DMSO solution containing Li+ cation. In relatively positive potential region, the mass per mol-electron (mpe) calculated from the slope of the relation between mass change and accumulated charge was 37 g/mol-e, indicating the deposition of lithium superoxide (LiO2) with the reaction of O-2 + e(-) + Li+ -> LiO2. As the potential became more negative, where superoxide (O-2(center dot-)) was formed in Li+ free solution, the mpe became much smaller (ca. 7 g/mol-e), suggesting the formation of O-2(center dot-), some of which were diffused away. (C) 2013 Elsevier B.V. All rights reserved.