The monomer 3-ethyl-1-vinyl-2-pyrrolidone (3) and the homopolymer poly(3-ethyl-1-vinyl-2-pyrrolidone) (5) have been synthesized. Polymer 5 is soluble in water and shows a critical temperature (T-c) of 27 degrees C. The presence of cyclodextrin causes a slight shift of the T-c. The lower critical solution temperature (LCST) could be varied between 27 and 40 degrees C by copolymerization with N-vinyl-2-pyrrolidone. A linear correlation between the T-c and the copolymer composition is observed.