Keywords: Ab initio calculations / Nitrogen heterocycles / Photolysis / Small ring systems / TautomerismIrradiation of the amino-4H-1,2,3-triazoles 7 (λ Ն 280 nm) affords molecular nitrogen and 3-amino-2H-azirines 8 in high yields. As shown on the basis of spectroscopic evidence, the photoproduct of 7a exists exclusively as 3-amino-2H-azirine 8a rather than in an equilibrium with the hypothetical iminoaziridine 10. Ab initio quantum chemical calculations up to the Gaussian-3 level were performed for pairs of 3-substituted 2H-azirines (1a, 3a, 5)/aziridines (2, 4, 6) and 1-substituted cyclopropenes (11, 13, 15)/cyclopropanes with exocyclic double bonds (12, 14, 16). The G3 energies of acyclic pairs of tautomers 17−22 and other compounds were calculated for comparison. Ring strain was assessed with the help [ ‡] The experimental results are part of the Dissertation by T.