The coexistence curves (T,n), (T,Φ), and (T,Ψ) (n, Φ, and Ψ are the refractive index, volume fraction, and effective volume fraction, respectively) for the ionic liquid microemulsion systems of {polyoxyethylene tert-octylphenyl ether (T-X100)+1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF 4 ])+cyclohexane} with various molar ratio (ω) of [bmim][BF 4 ] to T-X100 have been determined by measuring refractive indices at a constant pressure in the critical region. The critical temperatures (T c ) and critical volume fraction (Φ c ) were obtained for the ionic liquid microemulsions. The critical exponents were deduced precisely from the coexistence curves within about 1K below T c and the values were consistent with the 3D Ising value.