Room temperature ionic liquids (ILs) may have enormous potential as far as modifying the properties of aqueous surfactant solutions is concerned. A comparative study on the changes in the physicochemical properties of aqueous solution of a commonly used zwitterionic surfactant N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (SB-12) in the presence of up to 2 wt% of two ILs with same cation but different anions is undertaken using conductance, fluorescence spectroscopy, and dynamic light scattering. The two ILs used are 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) and 1-butyl-3-methylimidazolium hexafluorophosphate (Ibmim][PF6]). It is demonstrated that the addition of IL. alters the important properties of aqueous SB-12. Critical micelle concentration, aggregation number, micellar size, and microfluidity of micellar pseudo-phase change when the IL is added to aqueous SB-12. The extent to which these properties are altered is observed to be significantly more for [bmim[PF6] as compared to that for [bmim][BF4]. The results demonstrate the presence of electrostatic attractive interactions between the ions of ILs and the zwitterion of SB-12 to be the major reason for these observations. While the interaction between the IL cation and the anionic sulfonate termini of SB-12 is same for the two ILs, that between IL anion and cationic quaternary ammonium of SB-12 is proposed to be more efficient for [bmim][PF6] due to the bigger size of PF6-. Important role of IL anion in modifying properties of aqueous SB-12 is demonstrated. It is noted that the changes in the properties of aqueous surfactant systems upon IL addition depend strongly on the nature of the surfactant head group. (C) 2008 Elsevier Inc. All rights reserved.