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 0 degrees C 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 200 degrees C, while the secondary cyclization of the latter started above 250 degrees C. Spontaneous cyclodehydration was also observed for polyamide acid-amine 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.