The coexistence curves (T, n), (T, Phi), and (T, Psi) (n, Phi, and Psi 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][BF4]) + cyclohexane} with various molar ratio (omega) of [bmim][BF4] 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 (Phi(c)) were obtained for the ionic liquid microemulsions. The critical exponents were deduced precisely from the coexistence curves within about 1 K below T-c and the values were consistent with the 3D Ising value. (C) 2011 Elsevier Ltd. All rights reserved.