The effects of solvent and temperature on the rheo-optical behavior of hydroxypropylcellulose (HPC) solutions in water and acetic acid have been studied using flow birefringence, quiescent circular birefringence, and intrinsic viscosity. Measurements of steady state and transient flow birefringence suggest that the flexibility of the HPC molecule in water increases with increasing temperature, while it remains relatively constant in acetic acid. The relatively small temperature-induced changes of HPC are contrasted with the flow birefringence behavior of PBLG (poly(gamma-benzyl L-glutamate)) in the mixed-solvent system, dichloroacetic acid-1,2 dichloroethane, in which it undergoes a coil-helix transition. Results indicate that the effects of conformational changes on the dynamical behavior of HPC can be described in terms of a wormlike chain whose rigidity depends on the solvent as well as temperature and polymer molecular weight. A discussion of the possible roles of thermodynamic and structural effects on the molecular rigidity leads to the conclusion that structural effects are more likely.