The crystal structure of an ionic liquid, 1-isopropyl-3-methylimidazolium bromide ([i-C(3)mim]Br), Was studied using X-ray crystallographic analysis. The structures of three independent conformers of the isopropyl moiety were determined. Although the three conformers are classified into a group and their structures differ from the conformer described by Golovanov et al., the crystal structures are similar with respect to the inter-ionic interactions between [i-C(3)mim](+) and Br-. This polymorphic property of the ionic liquid is interpreted as arising because the enthalpy gain from the stronger Coulomb interactions determines a tight framework for the arrangement of Br- and the more positive moiety of the imidazolium ring. Thus, the isopropyl moiety is able to occupy the space within the framework in various conformations.