The solubility of N-isopropylacrylamide (NIPA) in water was found to discretely change at 25 degrees C. The highly concentrated NIPA solution separated into two solutions, the concentrations of which were higher than the solubility below 25 degrees C and lower than the solubility above 25 degrees C. The X-ray crystallographic analysis indicated that a NIPA crystal formed in the aqueous solution was H2O free. When the aqueous NIPA phase-separated solutions were cooled down to -90 degrees C at a rate faster than 35 degrees C/min, the glassy structure formed. On the other hand, the crystalline solid formation of NIPA and H2O were observed when the solutions were cooled to 50 degrees C at a slower rate than 3 degrees C/min. The DSC measurements of the phase-separated solutions revealed that the energy levels of NIPA were +15.2, +11.5, and -0.08 kJ/mol (regarding the crystalline solid state at 25 degrees C as the ground state) for the liquid state, the H2O-poor solution and the H2O-rich solution in the phase-separated state, respectively. The experimental results are explained in terms of the molecular assemblies of NIPA and H2O molecules in the solutions.