The phase behavior of hydroxypropylcellulose (HPC) in water has been studied in the temperature range from -3 to +46 degrees C by video-enhanced contrast (VEC) optical microscopy. In the low temperature range (below about 18 degrees C), a significant thermotropic effect was found in a narrow biphasic gap, with the critical HPC concentration for mesophase formation rapidly increasing as a function of temperature. The effect can be explained in terms of an increase of chain rigidity upon lowering the temperature, as estimated from intrinsic viscosity measurements. At higher temperatures (up to about 40 degrees C), phase separation takes place in-a wider region, which extends to a polymer concentration of about 65% wt at 25 degrees C. Upon a further increase in temperature, the material becomes white and turbid in a broad concentration range. When samples are examined at high magnification with the VEC optics, three distinct morphologies are observed in the turbid region, depending on concentration. At low concentrations, anisotropic spherulites are dispersed in an anisotropic medium. At high concentrations, isotropic inclusions of irregular shape are observed in an anisotropic continuous phase. A gel-like structure, characterized by a three-dimensional network, is displayed by solutions of intermediate concentrations. The phase diagram here obtained is compared to previous results on the HPC/water system and discussed in terms of the existing theories on liquid crystalline polymer thermodynamics.