The IR spectra of representative miscible aromatic polybenzimidazole/aromatic polyimide (PBI/PI) blends have been studied. Composition-dependent frequency shifts of up to 55 cm-1 in the N-H stretching band of PBI and up to 6 cm-1 in the carbonyl stretching bands of Pl were observed. These shifts were removed by thermally induced phase separation. The results suggest that the miscibility of these blends derives from specific intermolecular interactions involving the > NH and carbonyl groups.