To explore the role of additives in controlling the critical micellization temperature (CMT) of a block copolymer in aqueous solution, in the present work, a series of ionic liquids (ILs) containing the same cation, 1-butyl-3-methylimidazolium, bmim(+) and commonly used anions such as SCN-, BF4-, I-, Cl-, CH3COO- and HSO4-, were chosen for study with a triblock copolymer, poly(ethylene glycol)-blockpoly(propylene glycol)-block-poly(ethylene glycol), (PEG-PPG-PEG) in aqueous solution. The present results suggest that the ability of ILs for decreasing the polymer CMT is mainly a result of not only from charge and size of anions of the ILs, but also due to the weak ion-ion pair interactions within IL. The present study provides important information that can be helpful to tune the IL- or temperature-sensitive copolymer CMT and micelle shapes which are crucial for understanding the drug delivery mechanisms. (C) 2014 Elsevier Inc. All rights reserved.