The structure and phase transitions in the hydroxypropyl cellulose–mixed solvent water/ethanol system upon heating are studied. As the content of ethanol in the mixed solvent is increased, hydroxypropyl cellulose solutions undergo phase separation at higher temperatures. At the ethanol content above 50% there is no phase separation in the system upon heating to 400 К. The sizes of macromolecular associates in solutions are determined by the turbidity spectrum method.