Coexistences curves were reported for the liquid-liquid phase transition of binary solutions of the room temperature ionic liquid (RTIL) 1-octyl-3-methylimidazolium hexafluorophosphate ([C(8)mim][PF6]) in 1-butanol and 2-butanol. The critical amplitude B and the critical exponent beta corresponding to the coexistence curve were deduced and the values of p were found to be consistent with the 3D-Ising value. The experimental results are also analyzed to examine the asymmetry of the diameters for the coexistence curves. These asymmetries were discussed by the complete scaling theory. (C) 2013 Elsevier B.V. All rights reserved.