In this work, the physical and electrochemical behavior in a room temperature ionic liquid (RTIL), 1-butyl-1-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide (BMP-TFSI) has been investigated. Water was added to BMP-TFSI and the effect of added water on conductivity, electrochemical window (ECW) and oxygen reduction reaction (ORR) was explored. The results show that for water amounts up to 1.0 wt%, water addition to BMP-TFSI slightly increases the conductivity of BMP-TFSI due to "shear relaxation" in the water-ionic liquid mixture. As a result of an "impurity effect", an increase in water addition to BMP-TFSI brings about a reduction in the ECW for the mixture. The rotating ring-disk electrode (RRDE) technique has confirmed that the ORR process in as-received BMP-TFSI can be assigned to one-electron transfer: O-2 + e(-) <-> O-2(-center dot). The reversibility of the O-2/O-2(-center dot) redox couple is greatly reduced with the addition of water. The suppressed anodic peak is likely attributable to the fact that the superoxide readily reacts with water. In water added BMP-TFSI, the ORR peak occurs at a more positive potential with an increased current density. The improved ORR activity is attributed to a proton effect and the increase in electron transfer number of the ORR in water added BMP-TFSI. (C) 2014 The Electrochemical Society. All rights reserved.