The lyotropic behavior of the ternary system formed. by 1-tetradecy1-3-methylimidazolium chloride, 1-decanol, and water is investigated. A lamellar mesophase is formed for a wide range of compositions and is characterized by polarized optical microscopy, low-temperature scanning electron microscopy, small-and wide-angle X-ray scattering with synchrotron radiation, and differential scanning calorimetry. This phase presents onionlike structures. Two lamellar structures are formed: an La mesophase between 25 and SO degrees C, with an isobaric thermal expansivity of the bilayer thickness of -3.2 x 10(-3) K-1, and a lamellar gel phase, when the temperature decreases below 2S degrees C. This new medium is employed to perform in situ X-ray-initiated polymerization of N-isopropylacrylamide. When the monomer is incorporated in the lamellar structure, it is distributed between the water layer and bilayer interface and its polymerization can be followed by variations in the diffractograms with time.