Miscible blends of R3olybenzimidazole with an aromatic polyimide and a polyetherimide have been studied by high-resolution C CPMAS NMR in the solid state. The observed resonance signals in the spectra of individual components of the blends have been assigned, and these assignments have been supported by interrupted decoupling experiments. The blending of PBI with polyimide induces a broad­ening and a downfield shift of the aromatic polyimide phthalimide carbonyl resonance with respect to that of the pure material. This difference between the spectra of the blends and those of the respective mechan­ical mixtures has been interpreted to be the result of the formation of specific hydrogen bonds between the PBI imidazole amine function and the polyimide phthalimide carbonyl function. Miscibility in the blend of PBI with the aromatic polyimide has also been confirmed by a study of the proton rotating frame spin­lattice relaxation behavior.