We describe the lower critical solution temperature (LCST)-type phase behavior of poly(n-butyl methacrylate) (PnBMA) (M =13,000 and 48,000) dissolved in 1-alkyl-3-methylimidazolium bis{(trifluoromethyl) sulfonyl}amide ionic liquids (ILs). The temperature-composition phase diagrams of these PnBMA/IL systems are strongly asymmetric with the critical composition shifted to low concentrations of PnBMA. As the molecular weight increases from 13,000 to 48,000, the critical temperature decreases by 20 °C, and the critical composition shifts to a lower concentration. On the basis of the LCST of PnBMA, we designed a thermosensitive poly(n-butyl methacrylate)-poly(ethylene oxide) (PnBMA-PEO) diblock copolymer that exhibits a free chain/micelle transition in an IL as the temperature increases above the lower critical micellization temperature (LCMT). Furthermore, using IL blends as solvents, both the LCST of PnBMA and the LCMT of PnBMA-PEO can be tuned over a wide range by mixing two different alkyl methylimidazolium ILs without modifying the chemical structure of the polymers.