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 bis͑trifluoromethylsulfonyl͒imide ͑BMPTFSI͒ room-temperature molten salt ͑ionic liquid͒. The diffusion coefficient of O 2 was ͑1.8 ± 0.2͒ ϫ 10 −6 cm 2 s −1 at 25°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°C and decreased with an increase in temperature. The diffusion coefficient of O 2 − was 0.86 ϫ 10 −6 cm 2 s −1 at 25°C. The formal potential of O 2 /O 2 − couple was 1.13 V vs ferrocene/ferrocenium.