New pi-conjugated polymers constituted of a benzimidazole unit have been prepared. Palladium-catalyzed polycondensation between 4,7-dibromobenzimidazole bearing a hindered phenolic substituent (3,5-di-tert-butyl-4-hydroxyphenyl) and four kinds of diethynylaromatic compounds HC=C-Ar-C=CH (e.g., Ar = p-phenylene (p-Ph), m-phenylene (m-Ph), and pyridine-2,5-diyl (2,5-Py)) gives the corresponding poly(aryleneethynylene) (PAE) type polymers in high yields. The polymers with the p-Ph, m-Ph, and 2,5-Py groups have molecular weights of (0.4-3.3) x 10(4), as determined by GPC. The position of the pi-pi* absorption band as well as photoluminescence intensity of the polymers strongly depends on the kind of solvent. In general, the pi-pi* absorption peak of the PAE-type polymers with the p-Ph groups shifts to a shorter wavelength in acidic organic solvents, whereas that of the PAE-type polymers with the 2,5-Py unit shifts to a longer wavelength in the acidic solvent. Intensity of the photoluminescence increases in the acidic media. The polymers as well as a model compound are oxidized with PbO2. The oxidized products give rise to an IR peak at about 1680 cm(-1), which is assigned to nu(C=O) originated from resonance structures of the oxidized species, and their ESR signals appear at about g = 2.0 with a peak-to-peak line width of 8-11 G.