The micellar behavior of a nonionic surfactant, polyoxyethylene (20) sorbitan monolaurate (Tween-20) in different wt% of a hydrophilic ionic liquid, 3-methyl-1-pentylimidazolium tetrafluoroborate ([C(5)mim] [BF4) was investigated using techniques like fluoremetry and dynamic light scattering. Two fluorescence probes viz. pyrene and pyrene-1-carboxaldehyde were utilized. The critical micelle concentration (cmc) of Tween-20 was found to increase with increasing amount of [C(5)mim][BF4]. The significant increase in cmc and decrease in aggregation number (N-agg) indicates an "unfavorable" micellization within aqueous Tween-20 solution when [C(5)mim][BF4] is present. The increase in dipolarity upon [C-5mim][BF4] addition suggests increased water penetration due to the presence of [C(5)mim] [BF4] in the micellar pseudophase. Dynamic light scattering studies provide the evidence of micelle-like aggregates even in the presence of 2 wt% [C(5)mim][BF4]. (C) 2015 Elsevier B.V. All rights reserved.