The Cloud point phenomenon has been investigated for the solutions of polyoxyethylene (POE)type nonionic surfactants (C12E5, C12E6, C12E7. C10E6, and C14E6) in 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF(4)), a typical room temperature ionic liquid (RTIL). The cloud point. T, increases with the elongation of the POE chain, while decreases with the increase in the hydrocarbon chain length. This demonstrates that the solvophilicity/solvophobicity of the surfactants in RTIL comes from POE chain/hydrocarbon chain. When compared with an aqueous system, the chain length dependence of T-c is larger for the RTIL system regarding both POE and hydrocarbon chains; in particular, hydrocarbon chain length affects Tc much more strongly in the RTIL system than in equivalent aqueous systems. In a similar fashion to the much-studied aqueous systems, the micellar growth is also observed in this RTIL solvent as the temperature approaches Tc. The cloud point curves have been analyzed using a Flory-Huggins-type model based on phase separation in polymer solutions. (C) 2008 Elsevier Inc. All rights reserved.