Polybenzimidazole (PBI) and polysulfone (PS) do not form miscible blends; however, the tensile properties of the polymer blend in fibers, and the fiber process window, are strongly dependent on the presence of LiCl. The addition of LiCl opens the molecular chains and reduces dope viscosity. As a result, LiCl increases the stiffness of PBI/PS fibers while making them brittle. The process window for fabricating useful fibers seems to be narrowed by the presence of LiCl. PBI/PS fibers spun from a dope that does not contain LiCl have a broad process window and their overall fiber performance is superior. LiCl also reduced the degree of sulfonation during the stabilization process.