The reaction of different metasubstituted aromatic diamines with triethylorthoformate (TEOF) was investigated. Whereas the conversion of 1,3-phenylenediamine la resulted in an oligomeric polyformamidine, the formation of cyclic trimers was observed when nitrogen-containing aromatic diamines such as 2,6-diaminopyridine 1b, 2,4-diaminopyrimidine 1c, and 2,4-diamino-6-methyl-1,3,5-triazine 1d were used. The extent of cyclization depended on the number of nitrogen atoms in the aromatic ring and could be attributed to energetically favored conformations of the chain segments. In the case of Ib, cyclization was quantitative. The results were supported by a conformational analysis concerning the rotational barriers of the aromatic rings in the respective dimers. The reaction products were characterized by means of NMR and matrix-assisted laser desorption/ionization time-of-flight mass spectrometric investigations. In the reaction product of Id, strong hydrogen-bonded associates between the oligomers and the monomer were observed.