Polybenzimidazole diethyl phosphonates (PBIPEt) of high phosphorus content (as high as similar to 13%) have been synthesized by reacting poly[2,2'-(m-phenylene)-5,5'-bibenzimidazole] (PBI) with diethyl phosphite and a peroxide, in N,N-dimethylacetamide solution, via a free radical mechanism. The phosphonate ester groups are linked directly to the aromatic backbone thus establishing a strong -(O=)P-Ph-bond. The highly phosphonylated products dissolve in lower alcohols and the corresponding polyphosphonic acids (PBIPOH) dissolve in aqueous sodium hydroxide (pH similar to 7-8); membranes composed of either polymer are readily prepared. PBIPOH displayed high thermal stability as well as charge densities as high as similar to 9 mequiv of H+/g. Model compounds benzimidazole and 2,2'-diphenyl-5,5'-bibenzimidazole have been phosphonylated under similar conditions in order to resolve phosphonylation mechanism, and some aspects of a rather complicated oxidative substitution. Benzoic acid is a byproduct of reactions involving benzoyl peroxide supporting a mechanism that involves breakdown of the peroxide into radicals followed by reaction with the phosphite ester and subsequent attack of phosphorus centered radicals on an aromatic ring. In the case of PBI, phosphonylation appears to occur preferentially at the C-4 and C-7 positions of the benzimidazole rings (on the benzene ring adjacent to the fusion points) as well as on the m-phenylene rings ortho- to the linkage with benzimidazole. The observed substitution pattern is likely controlled by the resonance stability of the radical adduct.