Laser flash photolysis (LFP) of trans-2-tert-butylcyclopropyldiazirine (4) produces trans-2-tert-butylcyclopropylcarbene (5). Carbene 5 can be trapped with pyridine to form ylide 6. The rate of formation of ylide 6 was resolved and found to be linearly dependent on the concentration of pyridine. The lifetime of carbene 5 was determined to be 22 ns (pentane), 18 ns (acetonitrile and CF2ClCFCl2), 15 ns (cyclohexane), and 27 ns (cyclohexane-d(12)) at ambient temperature. The lifetime of carbene 5 in solution, at ambient temperature, is controlled substantially by reaction with solvent and only to a minor extent by rearrangement to cyclobutene 7. Photolysis of 4 (350 nm) in Freon-113 produces 3-tert-bulylcyclobutene (7) along with products derived from attack of the carbene on solvent. Photolysis of 4 in the presence of trapping agents (tetramethylethylene, cyclopentene, propylamine, trifluoroethanol) produces adducts with little or no corresponding decrease in the yield of 7. The preferred interpretation is that cyclobutene 7 is formed by a rearrangement of the excited state of the diazirine (diazirinyl biradical), and not by a carbenic process at 0 degrees C and lower temperatures. This mechanism is supported by product analysis of a non-nitrogenous cyclopropylcarbene precursor. At ambient temperature, cyclobutene 7 is formed by both diazirine excited state (diazirinyl biradical) and carbene rearrangement.