Cloud-point and binodal curves of the LCST type were obtained for aqueous solutions of a thermoresponsive polymer, poly [2-(2-ethoxy)ethoxyethyl vinyl ether], poly(EOEOVE). The cloud-point curve obtained was very flat except in a dilute region, that is the cloud-point temperature was insensitive to the polymer concentration, resembling the cloud-point curve for aqueous solutions of poly(N-isopropyl-acrylamide). On the other hand, the binodal curve obtained was parabolic, and located within the two-phase region of the cloud-point curve. Accompanied with the phase separation, a sharp endothermic peak was observed in a region including the cloud-point and binodal temperatures. The reciprocal of the osmotic compressibility partial derivative Pi/partial derivative c obtained by sedimentation equilibrium indicated that water changes from a good to poor solvent for poly(EOEOVE) with increasing temperature. Analyzing the partial derivative Pi/partial derivative c data by a thermodynamic perturbation theory, we determined the interchain interaction parameters, the hard-core diameter d and the depth epsilon of the square-well potential. Theoretical binodal and endothermic curves calculated by the perturbation theory using the estimated interaction parameters reproduced experimental ones semiquantitatively, but the theoretical binodal disagreed with the experimental flat cloud-point curve. The disagreement at high concentrations was in the opposite direction to that expected from the sample polydispersity in the molecular weight.(c) 2005 Wiley Periodicals, Inc.