The present invention provides for hybrid polybenzimidazole and polybenzimidazolone polymers which are characterized as containing both imidazole and imidazolone structural units in the polymer chain. These polymers are characterized as containing at least one recurring monomer unit containing the structure of the formula: ##STR1## wherein Ar is a tetravalent aromatic moiety derived from an aromatic tetraamine and Ar.sub.1 is a trivalent aromatic moiety derived from a trifunctional acid, an acid anhydride or amine-reactive derivatives thereof, said polymers further characterized by an inherent viscosity of at least about 0.01 dl/g measured as a 0.5% solution in N-methyl pyrrolidone at 25.degree. C.The polymers of this invention exhibit many of the advantageous properties of both polybenzimidazole and polybenzimidazolone polymers. They are characterized as being extremely resistant to chemical attack such as by solvents, acids and bases. Their high thermal stability and excellent electrical properties (low dielectric constant) render them useful in electrical and electronic applications.