The electrochemical reactions of O-2, CO2 and their mixture in three structurally different ILs (i.e. 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([BMIM][NTf2]), 1-butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide ([BdMIM][NTf2]) and 1-butyl-1methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([BMPY][NTf2]) were thoroughly investigated by in-situ electrochemical quartz crystal microbalance (EQCM). Compared with single electrochemical method, the QCM integrated with electrochemical method was shown to be significantly more sensitive and powerful for characterization of the subtle differences (mass change or viscoselastic change) on IL/electrode interface. It is found that the CO2 reduction in ILs is irreversible and forms CO2 center dot- adsorbate at electrode interface. With increasing concentrations of CO2, the reduction of oxygen is switched from a one electron process to overall two electron process, and forms adsorbed CO4 center dot- intermediate species. Even though the mechanisms of electrochemical reaction between CO2 and electrochemical generated superoxide radical (O-2(center dot-)) in three structurally different ILs are found to be similar, the simultaneously EQCM experimental results show that the different cation based ILs can modify the kinetics of the electrode reactions of O-2 and CO2 due to a competition between the ILs cation and CO2 to react with O-2(center dot-). The reactivity of O-2(center dot-) toward CO2 follows the order of the stability of the ILs cation under the O-2(center dot-) attack, i.e. [BMPY][NTf2] > [BdMIM][NTf2] > [BMIM][NTf2]. (C) 2013 The Electrochemical Society. All rights reserved.