We derived typical phase diagrams for aqueous solutions of methyl cellulose (MC) of different molecular weights via micro-differential scanning calorimetry, small-angle X-ray scattering, and visual inspection. The phase diagrams showed the cooccurrence of gelation and phase separation and qualitatively agreed with the theoretically calculated diagrams. The sol-gel transition line and phase separation line of a lower critical solution point type shifted toward lower temperatures and lower concentrations with an increase in the MC molecular weight. The sol-gel transition line intersected at a temperature higher than the critical point of the phase separation; therefore, both sol-gel phase separation and gel-gel phase separation were possible, depending on the temperature. Specifically, through visual inspection of a high molecular weight MC sample in the critical temperature region, we observed phase separation into two coexisting gels with different polymer concentrations. (C) 2000 John Wiley & Sons, Inc.