The aggregation behaviors of oleyl polyoxyethylene (10) ether. Brij 97, in room temperature ionic liquids, ethylammonium nitrate (EAN). pyrrolidinium nitrate ([Pyrr][NO3]), ethylammonium butyrate (EAB), 1-buty1-3-methylimidazolium hexafluorophosphate ([Bmim][PE6), and 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]), have been investigated. Only in the Brij 97/EAN binary system is the hexagonal liquid crystalline phase formed, and its ordering is found to decrease with increasing temperature. The lattice spacing values measured from the small-angle X-ray scattering (SAXS) shrink with reduction of ionic liquid content at room temperature. The general rules for aggregate formation in these ionic liquids are discussed and compared with that in water. A degraded ability to produce the ordered self-assembly of Brij 97 from H2O to EAN to [Bmim][PF6], [Bmim][BF4]. [Pyrr][NO3], and EAB is found and analyzed based on the molecular packing and Gordon parameters and also hydrogen-bonding or solvophobic interactions. Steady-shear rheoloizical measurements combined with the frequency sweep data indicate the highly viscoelastic nature of this liquid crystalline phase.