Surface tension measurements were carried out for the solutions of polyoxyethylene (20) sorbitan monolaurate (Tween 20) in 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF(4)) and hexafluorophosphate (bmimPF(6)) at various temperatures. Two transition points were found in the surface tension-concentration curves at each temperature. The freeze-fracture transmission electron microscopy revealed that two kinds of particles with different sizes are formed at the concentrations of each transition point. Thus, the surfactant concentrations of the two transition points are regarded as critical aggregation concentrations, CAC(1) and CAC(2) From the CAC values and their temperature dependence, we estimated the thermodynamic parameters of the aggregate formation, Delta G(agg)(0), Delta H-agg(0), and Delta S-agg(0). thermodynamic parameters related to CAC, are almost independent of temperature. On the other hand, as for the aggregate formation at CAC(2), a positive Delta S-agg(0) contributes to a neoative Delta G(agg)(0) at low temperature, while a negative Delta H-agg(0) contributes to a negative Delta G(agg)(0) at high temperature. The behavior of the thermodynamic parameters as a function of temperature, combined with the variation of H-1 NMR chemical shifts of the bmim(+) protons as a function of the surfactant concentration, demonstrated that the aggregates formed at CAC, are nanodroplets of Tween 20 segregated from the solution phase, while those formed at CAC(2) are similar to the usual surfactant micelles formed in aqueous solution.