AbstractBlends of a poly[2,2′‐(m‐phenylene)‐5,5′‐bibenzimidazole] (FBI) polymer with polyarylate (PA) yield films exhibiting synergistic mechanical properties. The break strength of the blended materials exceeded the strength of the stronger polymer in its 100% form. The molecular interaction and compatibility of PBI/PA have been investigated and confirmed by infrared, dynamic mechanical analysis and thermogravimetric analysis. For example, interactions were typically found between the carbonyls and the imidazole in these systems, and there was a shift of the carbonyl band to lower frequency. DMA study of a (40/60) PBI/PA film blend revealed the existence of twoTgvalues (e.g., 245 and 362°C). The shift of theTgfrom each individual component strongly suggested that FBI and PA interact on a molecular scale. Membranes made from a solution of a 60/40 (weight %) mixture of PBI and polyarylate exhibited excellent UF properties: a flux of 60–80 gfd at 50 psi and a rejection of 90–99% of cytochrome‐c (MW = 12,000). The HPLC results confirmed the red color species (Vitamin 12 and cytochrome‐c) were quantitatively removed. Further experiments indicated that the excellent UF properties were due to the novel combination of PBI and PA which offered a unique absorption and ionic‐ex