The glass transition behavior and viscoelastic properties of poly(methyl methacrylate) in mixtures with the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide are examined at compositions from 10 wt % polymer to pure polymer, over the temperature range from -180 to 230 °. Polymers of two different molecular weights (125 and 335 kg/mol) are studied. Glass transitions are analyzed by differential scanning calorimetry, and derivative heat flow curves are used to extract glass transition temperatures and breadths. Distinct composition dependences are observed for the polymer and ionic liquid components, with two apparent glass transitions at intermediate compositions. The glass transition breadths of the mixtures (∼30-70 °) are much broader than those of the pure components (<25 °). These results reflect distinct effective local compositions arising from the chain connectivity of the polymer component. The frequency-dependent dynamic moduli G′ and G′′ show a shift from unentangled to entangled behavior as concentration is increased from 10 to 20 wt % polymer. The application of timeerature superposition is successful over the full range of compositions, leading to master curves extending up to 11 orders of magnitude in reduced frequency. The plateau modulus (GN) exhibits a concentration dependence of GN ∼ c2.2, and analyses of the longest relaxation times and viscosity show the general trends expected for entangled solutions of increasing polymer concentration. Overall, ionic liquids are demonstrated to be effective model solvents for studying viscoelastic properties over wide temperature and composition ranges due to their nonvolatility and stability.