5-Phenyltetrazole le is an important source of phenylnitrene or the phenylnitrene radical cation (m/z 91) under thermal, photochemical, and electron impact conditions. Similarly, 3- or 4-(5-tetrazolyl)pyridines 12b,c yield pyridylnitrene radical cations 9a(center dot+)-(m/z 92) upon electron impact. In contrast, 2-(5-tetrazoly1)-pyridine 12a(center dot+) generates 2-pyridyldiazomethane 24(center dot+) and 2-pyridylcarbene 26(center dot+) radical cations (m/z 119 and 91) upon electron impact. The 2-pyridylcarbene radical cation undergoes a carbene-nitrene rearrangement to yield the phenylnitrene radical cation. Calculations at the B3LYP/6-311G(d,p) level have revealed facile H-transfer from the tetrazole to the pyridine ring in 2-(5-tetrazolyOpyridine, 12a(center dot+) -> 21(center dot+), taking place in the radical cations. Subsequent losses of N-2 generate the pyridinium diazomethyl radical 22(center dot+)or pyridinium-2-carbyne 23(center dot+). These two ions can isomerize to 2-pyridyldiazomethane 24(center dot+) and 2-pyridylcarbene 26(center dot+) the latter rearranging to the phenylnitrene radical cations 9a(center dot+). C-13-labeling of the tetrazole rings confirmed that 2-(S-tetrazolyl)pyridine 12a generates 2-pyridylcarbene/phenylnitrene radical cations retaining the C-13 label, but 4-(5-tetrazolyl)pyridine 12c generates 4-pyridylnitrene 18c(center dot+), which has lost the C-13 label. 2-Pyridylcarbene/phenylnitrene radical cations (m/z 91) also constitute the base peak in the mass spectrum of 1,2,3-triazolo[1,5-alpha]pyridine 34. Similarly, 4pyridylnitrene radical cation 18c(center dot+) or its isomers (m/z 92) is obtained from 1,2,3-triazolo[1,5-alpha]pyrazine 36. Several other alpha-heteroaryltetrazoles behave in the same way as 2-(5-tetrazolyl)pyridine, yielding heteroarylcarbene/arylnitrene radical cations in the mass spectrometer, and this was confirmed by C-13-labeling in the case of 1-(5-tetrazolyl)isoquinoline 42-C-13. In general, 5-aryltetrazoles generate arylnitrene radical cations under electron impact, but alpha-heteroaryltetrazoles generate alpha-heteroarylcarbene radical cations.