Reactions of 5-amino-alpha-imino-1H-imidazole-4-acetonitriles 1 with alkyl and aryl isocyanates led to efficient syntheses of 5'-amino-5-imino-4,4'-bi-1H-imidazol-2-ones 3 formed by intramolecular cyclization of the corresponding 5-amino-alpha-(N-alkyl/arylcarbamoyl)imino-1H-imidazole-4-acetonitriles 2. The cyclization occurs only slowly in solution but is considerably accelerated by the addition of a catalytic amount of DBU (1,8-diazabicyclo[5.4.0]undec-7-ene). The reaction of the N-arylamidine 6b, the synthetic precursor of the imidazole 1b, with benzyl isocyanate also led to the formation of 4,4'-bi-1H-imidazol-2-one 3b in quantitative yield. The imidazole intermediate 2b has been isolated and found to be identical with the compound obtained by reaction of the imidazole 1b and benzyl isocyanate. The N-arylamidine 6c (R = 4-NCC(6)H(4)) reacted with benzyl isocyanate in a similar way, but the electrophilicity of the amidine carbon atom resulted in rapid hydrolysis of the intermediate 7c leading ultimately to the isolation of the urea 9. The N-alkylamidines 6a and 6d behaved differently in their reaction with benzyl isocyanate, and the major product isolated in these reactions is again the urea 9.