An upper critical solution temperature (UCST)-type phase separation in water was achieved using well-defined polymeric ionic liquids (ILs) with imidazolyl groups in their side chains, prepared based on living cationic polymerization using a cationogen/Et1.5AlCl1.5 initiating system with 1,4-dioxane as an added base. Aqueous solutions of the polymers with tetrafluoroborate as counteranions showed sharp and reversible UCST-type phase separation at 5-15 degrees C. The effect of polymer concentration, chain-end groups, and molecular weight on the phase separation temperature suggests that the phase separation resulted from interpolymer electrostatic interactions. Other polymeric Its with SbF6- also showed a lower critical solution temperature-type phase separation in various organic solvents.