1,4-Dialkyl-5-(N-alkylimino)-4,5-dihydro-1H-tetrazoles were prepared in high yields by deprotonation with sodium hydride of 1,4-dialkyl-5-(N-alkylamino)tetrazolium salts that were adorned with two or three different alkyl groups, including methyl, trideuteriomethyl, and tert-butyl groups. Direct irradiation (lambda > 255 nm) at -60 degrees C yielded molecular nitrogen and mixtures of 1,2-dialkyl-3-(N-alkylimino)diaziridines (83-87%) along with carbodiimides (13-17%) arising by 1,3-dipolar cycloreversion. The missing 1,3-dipoles, alkyl azides, did not survive photolysis. Each member of a pair of isotopomers and of a pair of isomers, and an iminodihydrotetrazole, whose three nitrogens were tagged, yielded a characteristic mixture of three isomeric iminodiaziridines that allowed the mode of formation to be deduced. The results are interpreted in terms of photodenitrogenation of the iminodihydrotetrazoles to furnish diastereomeric singlet triazatrimethylenemethane diradicals that retain the inherited configurations before ring closure to iminodiaziridines, presumably in two steps via mono-orthogonal diradicals.