Abstract: A miscible blend comprising from about 5 to about 95 weight percent of a polybenzimidazole and from about 95 to about 5 weight percent of a polyamide-imide having fluorine-containing linking groups, total weight percent of the two polymers being 100. A process for preparing the blends comprises dissolving in a mutual solvent from about 5 to about 95 weight percent of the aromatic polybenzimidazole to about 95 to about 5 weight percent of the polyamide-imide having fluorine-containing linking groups, based on the total weight of those polymers and subsequently evaporating the solvent, thereby producing a dried blend of a polybenzimidazole and a polyamide-imide having fluorine-containing linking groups. The blends, particularly in the form of films, fibers or fibrets, may be post-treated with heat or sulfuric acid in order to minimize their shrinkage when subsequently subjected to heat and in order to increase their resistance to solvents and acid.