Irradiation of 2-adamantane-2,3′-[3H]-diazirine (1) in isooctane at room temperature gives two primary photoproducts, 2-diazoadamantane and 2-adamantylidene (Ad:), with quantum yields of about 0.5 each. The fluorescence quantum yield of 1 is small, φ f (1) ≈ 10 −4 . The reactivity of Ad: was studied by nanosecond laser flash photolysis; it is best described as that of a carbene equilibrating between the singlet ground state and a low-lying triplet state. Ad: reacts with the precursor diazirine 1, pyridine, acetonitrile, tetramethylethylene, amines, water, and acetic acid with rate constants approaching the diffusion-controlled limit, as well as with molecular oxygen (k O 2 = 2.3 × 10 9 M −1 s −1 ). Reaction of Ad: with 1 (k x = 2.3 × 10 9 M −1 s −1 ) gives an ylide, λ max = 290 nm, which forms adamantanone azine as a stable product by a reaction having activation parameters E a = 15.5 kcal mol −1 and A = 2 × 10 12 s −1 . At low concentrations of 1 in thoroughly dry, degassed solvents, the lifetime of Ad: reaches ca. 225 ns in isooctane, 500 ns in perfluorodecalin, and 700 ns in benzene. Diazoadamantane forms azine by a second-order reaction with a half-life of several hours at 10 −4 concentration.