The AB-monomer, 3,4-diaminobenzoic acid dihydrochloride, was recrystallized from an aqueous hydrochloric acid solution and used to synthesize high-molecular-weight poly(2,5-benzimidazole) (ABPBI). ABPBI/carbon nanotube (CNT) composites were prepared via in situ polymerization of the AB-monomer in the presence of single-walled carbon nanotube (SWCNT) or multiwalled carbon nanotube (MWCNT) in a mildly acidic polyphosphoric acid. The ABPBI/SWCNT and ABPBI/MWCNT composites displayed good solubility in methanesulfonic acid and thus, uniform films could be cast. The morphology of these composite films was studied by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. The results showed that both types of CNTs were uniformly dispersed into the ABPBI matrix. Tensile properties of the composite films were significantly improved when compared with ABPBI, and their toughness (similar to 200 MPa) was close to the nature's toughest spider silk (similar to 215 MPa). The electrical conductivities of ABPBI/SWCNT and ABPBI/MWCNT composite films were 9.10 x 10(-5) and 2.53 x 10(-1) S/cm, respectively, whereas that of ABPBI film was 4.81 X 10(-6) S/cm. These values are similar to 19 and 52,700 times enhanced by the presence of SWCNT and MWCNT, respectively. Finally, without acid impregnation, the ABPBI film was nonconducting while the SWCNT- and MWCNT-based composites were proton conducting with maximum conductivities of 0.018 and 0.017 S/cm, respectively. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48:1067-1078, 2010