The electrochemical behavior of oxygen (O-2)/superoxide ion (O-2(-)) couple was investigated with the aid of the ultramicroelectrode technique in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMPTFSI) room-temperature molten salt (ionic liquid). The diffusion coefficient of O-2 was (1.8 +/- 0.2) X 10(-6) cm(2) s(-1) at 25 degrees C. The activation energy of the diffusion process of O-2 was estimated to be 27 kJ mol(-1) from the temperature dependence of the diffusion coefficient. The solubility of O-2 in BMPTFSI was 14 mmol dm(-3) at 25 degrees C and decreased with an increase in temperature. The diffusion coefficient of O-2(-) was 0.86 X 10(-6) cm(2) s(-1) at 25 degrees C. The formal potential of O-2/O-2(-) couple was 1.13 V vs ferrocene/ferrocenium. (c) 2005 The Electrochemical Society. All rights reserved.