The condensation reactions of the structurally similar monomers 1,3-phenylenediamine (1a) and 2,6-diaminopyridine (1b) with N,N-carbonyldiimidazole have been compared. Whereas the reaction of la resulted in an amino terminated oligomeric polyurea, a mixture of two macrocyclic trimers and one macrocyclic tetramer was obtained in the case of 1b. The three components of the mixture could be isolated by extraction with DMSO at different temperatures. The structure of the macrocyclic compounds was investigated by means of NMR and matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy.