The structure and lower critical solution temperature (LCST) phase behavior of well-defined poly(benzyl methacrylare) (PBnMA) solution using 1-ethy1-3-methylimidazolium bis(trifluoromethanesulfonyl) amide, [C(2)mim][NTf2] ionic liquid (IL) as a solvent have been studied by dynamic light scattering (DLS) and small-angle neutron scattering (SANS) at various temperatures. The SANS profiles observed for fully deuterated IL ([C(2)mim]-d(11)[NTf2]) containing PBnMA were kept practically unchanged in the temperature range between 298 and 363 K, while they suddenly changed at 363 K. This indicates that the LCST behavior of PBnMA IL solution is a first-order phase transition, which is consistent with the DLS results. The SANS profiles below 363 K were well represented by the theoretical Debye scattering function with inter-molecular interaction and the radius of gyration, R-g was estimated to be almost constant, i.e., similar to 45 angstrom. The SANS result obtained here was compared with those in aqueous PNIPAm solutions as a typical LCST system, and some differences between IL and aqueous solution systems are pointed out. It is found that thermodynamic quantities (Delta H(demi)x, Iota S-demix and Delta G(demix)) from the homogeneous solution to the phase separation states strongly depend on the solvation of the PBnMA polymer by the IL ([C(2)mim] cation and [NTf2] anion). We propose an LCST phase separation mechanism in the polymer-IL solution. (C) 2011 Elsevier Ltd. All rights reserved.