The model reaction between phthalic anhydride and 1,2-diaminobenzene was studied under conditions analogous to those of the low-temperature polycondensation of aromatic dianhydrides with bis(o-diamines) in amide solvents followed by thermal cyclodehydration in condensed state to form ladder polyimidazopyrrolones (polypyrrones). The intermediate N-(o-aminophenyl)phthalamic acid was found to undergo spontaneous cyclodehydration to give N-(o-aminophenyl)phthalimide and 2-( o-carboxy-phenyl)benzimidazole. The reaction occurred at ambient temperature in the presence of water or alcohol without using a dehydration agent. The yield of imide-amine and/or carboxy-benzimidazole depended on the temperature of the condensation reaction. Temperatures below 0ЊC appeared to favor the formation of the carboxy-benzimidazole. Thermal cyclization of N-(o-aminophenyl)phthalamic acid passed through the same intermediates: imide-amine and carboxy-benzimidazole. The former converted to corresponding pyrrone above 200ЊC, while the secondary cyclization of the latter started above 250ЊC. Spontaneous cyclodehydration was also observed for polyamide acidamine precursors obtained by low-temperature polycondensation of pyromellitic dianhydride with 3,3 -diaminobenzidine. The prepolymer solutions in DMF diluted with water at room temperature turned to a gel after 48-72 h. A spectroscopic study of the resulting polymers indicated the presence of significant amounts of imide cycles.