Micellization behaviour of cetyltrimethylammonium bromide (CTAB) in the presence of ionic liquid (IL), 1-butyl-2,3-dimethylimidazolium bromide [bdmim][Br] in aqueous medium has been investigated by conductance, fluorescence, H-1 NMR, dynamic light scattering (DLS) and transmission electron microscopy (TEM) techniques. The temperature dependence of critical micelle concentration (cmc) obtained from conductivity measurements at different temperatures (298.15, 308.15, and 318.15) K has been used to calculate various thermodynamic parameters of micellization and the obtained parameters are further used to understand the surfactant-IL interactions for the studied (0-0.2) wt% of IL. Further fluorescence technique has been used to obtain the cmc and aggregation number from (0-30.0) wt% of added IL in the surfactant solution using different probes at 298 K. To understand the variation in micellar size at different wt% of IL, DLS and TEM techniques have been employed. Interestingly, due to presence of IL in surfactant solution in either low or high proportion, micellar size increases. IL effectively enhances the surface activity of surfactant solution until 2.0 wt% of added IL but above this wt% it starts increasing cmc. The crucial role of electrostatic interactions is clearly implied by H-1 NMR studies. (C) 2015 Elsevier B.V. All rights reserved.