The miscibility of polybenzimidazole {poly[2,2＇-(m-phenylene)-5,5＇-bibenzimidazole], (PBI)} and a polyimide {poly[2,2＇-bis(4-(3,4-dicarboxyphenoxy)phenyl)propane-m-phenylenediimine] (PI)} was studied in the entire composition range. Films were cast from solution by a procedure developed in a preliminary work. The blends were studied by FT-IR spectroscopy, thermal and thermomechanical methods, and by measuring vapour absorption at an ambient temperature. Direct relationships were found between the strength of hydrogen-bonding of the components, the glass transition temperature of the blends, and the solvent diffusion rate (water and 1,2,4-trichlorobenzene). The results of the experiments indicate partial miscibility between the two polymers. Phase inversion occurs between 0.4 and 0.6 PI content.