We demonstrate that a microelectrode can be used as a diagnostic tool to optimize the properties of electrolytes for non-aqueous Li-air batteries, and to elucidate the influence of ion-conducting salts on O-2 reduction reaction mechanisms. Oxygen reduction/evolution reactions on carbon microelectrode have been studied in dimethyl sulfoxide-based electrolytes containing Li+ and tetrabutylammonium((C4H9)(4)N+) ions. Analysis of chronoamperometric current-time transients of the oxygen reduction reactions (ORR) in the series of tetrabutylammmonium (TBA) electrolytes, TBAPF(6), TBAClO(4), TBACF(3)SO(3), TBAN(CF3SO2)(2) in DMSO revealed that the anion of the salt exerts little influence on oxygen transport. Whereas steady-state ORR currents(sigmoidal-shaped) were observed in TBA-based electrolytes, peak-shaped current-voltage profiles were seen in the electrolytes containing their Li salt counterparts. The latter response results from the combined effects of the electrostatic repulsion of the superoxide intermediate as it is reduced further to peroxide (O-2(2-)) at low potentials, and the formation of passivation films at the electrode. Raman spectroscopic data confirmed the formation of Li2O2 and Li2O on the microelectrode surface at different reduction potentials in Li salt solutions. Out of the four lithium electrolytes, namely LiPF6, LiClO4, LiCF3SO3, or LiN(CF3SO2)(2) in DMSO, the LiCF3SO3/DMSO solution revealed the most favorable ORR kinetics and the least passivation of the electrode by ORB. products. (C) 2014 The Electrochemical Society. All rights reserved.