Laser flash photolysis (266 nm) of diphenylphosphoryl azide 1 produces singlet diphenylphosphorylnitrene 2s (not observed), which can either react with solvent or relax to form the lower energy triplet nitrene 2t. Triplet 2t absorbs at 345 nm in chloroform, cyclohexane, 1,2-dichloroethane, dichloromethane, ethyl acetate, 1,1,2-trichlorotrifluoroethane, and tetrahydrofuran, but the intensity and shape of the absorption band is solvent-dependent. The triplet lifetime is several tens of microseconds in the aforementioned solvents. The magnitude of 1/tau is linearly dependent on the concentration of diphenylphosphoryl azide 1(k(1)(3) = 1.2 X 10(6) M-1 s(-1)) in methylene chloride. The extrapolated lifetime of 2t in the absence of precursor is 500 mus in methylene chloride at ambient temperature. Thus, the long triplet lifetime in the aforementioned solvents is controlled by nitrene reaction with precursor. The one exception is methanol as solvent for which the triplet lifetime is only 6 mus. The reciprocal of the lifetime of 2t is linearly dependent on the concentration of tris-trimethylsilylsilane ((Me3Si)(3)SiH), and the absolute rate constants to hydrogen transfer (k(Si-H)(3)) are 3.4 x 10(5) M-1 s(-1) in 1,2- dichloroethane and 3.1 X 10(5) M-1 s(-1) in 1,1,2-trifluorotrichloroethane. A polar effect on k(SiH)(3) increases the rate constant to reaction with the silane in methanol and shortens the lifetime of 2t in this solvent. (MeSi)(3)SiH reduces the yield of triplet 2t (phi) by the trapping of singlet nitrene 2s. Stern-Volmer analyses reveal that the singlet lifetime 2s is 1, 1, and 4 ns in 1,2-dichloroethane, methanol, and 1,1,2-trifluorotrichloroethane, respectively, assuming that k(SiH)(1) is 5 x 10(9) M-1 s(-1). The rapid rate of nitrene intersystem crossing is attributed to a N-O bonding interaction in the singlet nitrene, which results in a closed-shell electronic structure of the singlet.