Modification of important physicochemical properties of aqueous surfactant solutions can be achieved by addition of environmentally benign room temperature ionic liquids (ILs). While low aqueous solubility of "hydrophobic" ILs limits the amount of IL that may be added to achieve desired changes in the physicochemical properties, hydrophilic ILs do not have such restrictions associated to them. Alterations in the key physicochemical properties of aqueous solutions of a common nonionic surfactant Triton X-100 (TX100) on addition of up to 30 wt % hydrophilic IL 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim] [B F4]) are reported. The presence of micellar aggregates in as high as 30 wt % [bmim] [B F4] -added aqueous TX100 solutions is established by dynamic light scattering and fluorescence probe behavior. Increasing the concentration of [bmim] [B F4] results in decrease in average micellar size and aggregation number and increase in critical micelle concentration, indicating an overall unfavorable aggregation process. Increase in the dipolarity and the microfluidity of the probe cybotactic region within the palisade layer of the micellar phase upon [bmim] [B F4] addition implies increased water penetration and the possibility of TX100- [bmim] [B F4] interactions. While the changes in some of the physicochemical properties indicate the role of [bmim] [B F4] to be similar to a cosurfactant, the IL acts like a cosolvent as far as changes in other properties are concerned. Effectiveness of IL [bmim] [B F4] in modifying physicochemical properties of aqueous TX100 is demonstrated. © 2007 American Institute of Physics.