Three samples of poly[2,2′-(o-phenylene]-5,5′-bibenzimidazole] (PBI) were prepared by condensation of 3,3′-diaminobenzidine (DAB) with, respectively, phthalic anhydride in p-cresol under NZ, with phthaloyl chloride in a xylene-water system containing carbonate, and with phthalimide in polyphosphoric acid. Solutions of these PBI samples in formic acid exhibited an anomalous dependence of the reduced viscosity on concentration. This poly-electrolyte behavior of the polymer solution was analyzed by application of empirical relations. The intrinsic viscosity of the PBI samples was measured in formic acid containing 1% (w/v) KBr. A solution of the N-methyl derivative of a PBI (melt) sample showed normal behavior in a viscometric study. The results of DTA of three PBI samples, one N-methyl PBI (melt) sample, and three model compounds are compared and analyzed with a view to estimating the energy of activation and the order of degradation reaction.