Flash vacuum thermolysis (FVT) of phenyl azide 29 as well as precursors of 2-pyridylcarbene 34 and 4-pyridylcarbene 25 affords phenylnitrene 30 (labeled or unlabeled), as revealed by matrix isolation electron spin resonance spectroscopy. FVT of 1-(13)C-phenyl azide 29 affords 1-cyanocyclopentadiene (cpCN) 32, which is exclusively labeled on the CN carbon, thus demonstrating direct ring contraction in phenylnitrene 30 without the intervention of cycloperambulation and 1,3-H shifts. However, the cpCN obtained by rearrangement of pyridyl-2-((13)C-carbene) 34 carries (13)C label on all carbon atoms, including the CN carbon. Calculations at the B3LYP/6-31G* level and in part at the CASSCF/6-31G* and CASPT2/cc-pVDZ//CASSCF(8,8)/cc-pVDZ levels support a new mechanism whereby 2-pyridylcarbene rearranges in part via 1-azacyclohepta-1,2,4,6-tetraene 36 to phenylnitrene, which then undergoes direct ring contraction to cpCN. Another portion of 2-pyridylcarbene undergoes ring expansion to 4-azacyclohepta-1,2,4,6-tetraene 42, which then by trans-annular cyclization affords 6-azabicyclo[3.2.0]cyclohepta-1,3,5-triene 43. Further rearrangement of 43 via the spiroazirine 44 and biradical/vinylnitrene 45 affords cpCN with the label on the CN group. An analogous mechanisms accounts for the labeling pattern in fulvenallene 60 formed by ring contraction of 1-(13)C-phenylcarbene 59 in the FVT of 1-(13)C-phenyldiazomethane 58.