The interaction of b-casein micelles (b-CMs) with imidazolium based ionic liquid (IL) surfactant ([C 12 mim]Br) has been studied using turbidity, isothermal titration microcalorimetry, fluorescence spectroscopy, dynamic light scattering (DLS) and transmission electron microscopy (TEM) measurements. Below c 1 , the individual [C 12 mim]Br monomers bind onto the b-CM shell close to the hydrophobic core to form a b-CM-[C 12 mim]Br (monomer) complex. The hydrophobic tail of [C 12 mim]Br leads to a significant decrease in the environmental polarity of b-CM. Just over c 1 , [C 12 mim]Br molecules aggregate into micelle-like aggregates on the micellar shell. This leads to the collapse of the N-terminal of b-casein and strengthens the hydrophobicity of the protein molecules, resulting in a more compact structure of b-CM. With a continuous increase in[C 12 mim]Br concentration, b-CMs associate with each other into a network-like structure. Beyond c 3 , the net positive charges on the complexes, owing to the binding of more cationic surfactant molecules, lead to redissociation of the complexes, corresponding to the formation of the new nano-sized b-CM-[C 12 mim]Br complexes. All the b-casein molecules are saturated by [C 12 mim]Br aggregates above c s , and free [C 12 mim]Br micelle-like aggregates appear in the bulk phase above critical aggregation concentration (cac). From a combination of experimental results and discussion on various interactions in the b-CM-[C 12 mim]Br system, the hydrophobic interaction between the hydrophobic tail of ILs and the hydrophobic domain of b-CM, the electrostatic attraction between [C 12 mim] + and negative charged amino acid residues on b-CM shell, and the hydrogen bonding between [C 12 mim] + and carboxylic moiety on b-CM shell are the main forces for [C 12 mim]Br binding to b-CM. Modifications in the physicochemical properties of b-CM upon the addition of [C 12 mim]Br will expand and enhance the overall capabilities and applications of b-CM.