The solubility and phase behavior of linear polymethacrylate polymers, primarily poly(phenylalkyl methacrylate)s, in imidazolium-based ionic liquids (ILs) were systematically studied by changing the structure of each component. Solutions of polymethacrylates in 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C(n)mim] [NTf2]) showed lower critical solution temperature (LCST) phase behavior, and the phase separation temperature (T-c) could be varied by selecting an appropriate combination of a polymer and an IL. An increase in alkyl chain length between the phenyl and ester groups in the polymer side chain decreased the T-c; alternatively, substitution of the imidazolium cation with a longer alkyl chain increased the T-c. When the same anion was used, the miscibility of the polymer/IL system was mainly determined by the alkyl chain length. T-c could also be varied by mixing two ILs in an appropriate ratio. In addition, the kinetics of the reversible phase transition phenomena exhibited by these polymers were examined. Redissolution kinetics were largely controlled by the magnitude of the difference between T-c and the glass transition temperature (T-g) of the polymer (T-c - T-g), in addition to the mutual affinity between the polymer and the IL.