Electrochemical quartz crystal microbalance (EQCM), which can detect surface mass change inng scale, was employed to investigate the deposition of Li–O2 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 37g/mol-e, indicating the deposition of lithium superoxide (LiO2) with the reaction of O2 +e− +Li+ →LiO2. As the potential became more negative, where superoxide ( O 2 - ) was formed in Li+ free solution, the mpe became much smaller (ca. 7g/mol-e), suggesting the formation of O 2 - , some of which were diffused away.