In this work, we have investigated the conductivity, electrochemical window (ECW) and oxygen reduction reaction (ORR) in various bis(trifluoromethanesulfonyl)imide (TFSI)-based room temperature ionic liquids (RTILs). In an attempt to improve ORR activity in RTILs, water and/or dimethyl sulfoxide (DMSO) are added to one TFSI-based RTIL, 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (BMP-TFSI), and the effect of additives on its conductivity, ECW and ORR. is explored. The results show that as-received TFSI-based ionic liquids (ILs) have a low conductivity due to their large constituent ions. The addition of water and/or DMSO has considerable effect on the conductivity of the IL mixture. The studied TFSI-based RTILs have wide ECWs and the presence of water and/or DMSO causes an apparent narrowing of the observed potential window. For all TFSI-based ILs, the oxygen redox peaks can be assigned to an O-2/O-2(-center dot) one-electron transfer process. By adding water, the O-2/O-2(-center dot) redox potential shifts in a more positive direction in BMP-TFSI; e.g., about 0.27 V more positive in BMP-TFSI with 1 wt% water addition, due to its high acceptor number (AN) and the proton effect. The addition of DMSO also contributes to the improvement in ORR activity in BMP-TFSI through enhanced conductivity and redox reversibility. (C) 2014 The Electrochemical Society. All rights reserved.