The phase diagram of the system hydroxypropylcellulose (HPC)-H2O has been investigated using differential scanning calorimetry (DSC), X-ray diffraction, optical microscopy and H-1 (NMR) nuclear magnetic resonance spectroscopy. A peritectic line at higher temperatures has been found similar to the lytropic systems described before. This phase transition, from a solution consisting of solvate aggregates and free solvent, into a crystal solvate-isotropic phase system is completely reversible. The formation of crystal solvate phases is independent of the water content of the system until the system transforms into a thermotropic system. The solvent molecules are strongly associated with the macromolecules.