Abstract—: A homologous series of poly(2,5(6)-benzimidazole) (ABPBI) samples in a mixed DMSO/MeOH/KOH solvent is studied by viscosimetry and dynamic light scattering. The dissolution of ABPBI is found to occur due to the deprotonation of NH and the formation of an ABPBI anion. In the mixed solvent, this polymer demonstrates behavior typical of polyelectrolytes. This polyelectrolyte effect is neutralized by increasing the concentration of KOH. The empirical Mark-Kuhn–Houwink (MKH) equations are obtained, where the indices of power turn out to be noticeably higher than those for Gaussian coils in the ideal solvent. The conformational parameters of ABPBI are analyzed in detail using theories taking into account both bulk interactions and polymer-chain draining effects.