Solid-state characterization of poly(L-histidine) was obtained via differential scanning calorimetry, thermogravimetric analysis, optical microscopy, and infrared spectroscopy. The glass transition temperature of poly(L-histidine) is 169 degrees C. This thermal transition has not been reported previously. Poly(L-histidine)＇s T-g increases when complexes are produced with the following divalent transition metal chlorides: cobalt chloride hexahydrate, nickel chloride hexahydrate, copper chloride dihydrate, and anhydrous zinc chloride. At 10 mol % salt, nickel chloride increases T-g by 69 degrees C. The enhancement in poly(L-histidine)＇s T-g correlates well with ligand field stabilization energies for pseudo-octahedral d(n) complexes (n = 7, 8, and 10) from the first row of the d-block. However, d(9) copper(II) complexes do not conform to this empirical correlation. Infrared spectroscopic evidence indicates that these metal chlorides form complexes with the imidazole ring in the histidine side group and the amide group in the main chain of the polymer.