Phase diagrams of two ionic liquids: hydrophobic 1-butyl-3-methylimidazolium hexafluorophosphate (bmim-PF6) and relatively hydrophilic 1-butyl-3-methylimidazolium tetrafluoroborate (bmim-BF4) in aqueous solutions of Brij 97 were determined at 25 degrees C. Two hexagonal liquid crystalline phases formed in bmim-PF6-and bmim-BF4-containing ternary systems were investigated by means of small-angle X-ray scattering (SAXS) and rheological techniques, with comparison of composition and temperature effects. From analysis of the SAXS data, bmim-PF6 is dominantly penetrated between the oxyethylene chains of surfactant molecules, whereas bmim-BF4 is mainly located in the water layer of hexagonal phases. The strength of the network of hexagonal phase formed in the Brij 97/wateribmini-BF4 system is appreciably stronger than that of the Brij 97/water/bmim-PF6 system, indicated by the smaller area of the surfactant molecule at the interface and the higher moduli (G', G"). Temperature has a converse effect on the lattice parameters of the two hexagonal phases.