Phase behavior and ordering characteristics were reported for poly(gamma-benzyl L-glutamate) (PBLG) in a mixed solvent containing a denaturant acid, dichloroacetic acid (DCA), + dichloroethane (DCE). The solutions of PBLG prepared so as to be liquid crystal at room temperature were observed to undergo a reentrant isotropic transition at low temperature where conformational disordering of PBLG molecules takes place. The deuterium quadrupolar splitting data obtained by the H-2 NMR method were analyzed to elucidate the conformation and orientational order parameter of the ct-helix in the liquid crystalline phase. The conformation of PBLG molecules in the isotropic state was estimated from the observed C-13 NMR chemical shift. The molecular information gained through these experiments was implemented into the theoretical expression set forth by Flory and Matheson. The phase diagrams calculated on this basis have been compared with those observed. The theory has provided a reasonable picture for the high- and low-temperature phase transitions of the PBLG/DCA/DCE system.