Laser flash photolysis (LFP, Nd:YAG laser, 35 ps, 266 nm, 10 mJ) of phenyl azide (PA) releases open-shell singlet phenylnitrene ((PN)-P-1). (PN)-P-1 has a sharp absorption band at 350 nm and a very weak absorption band, at 540 nm. These bands are assigned as n(z)-n(y) and a mixture of 52% pi-n(x) and 28% n(x)-pi* transitions, respectively, on the basis of CASSCF and CASPT2 calculations. A CASPT2 calculation of the spectrum of triplet phenylnitrene ((PN)-P-3) is in very good agreement with the experimental spectrum. A sharp absorption band of (PN)-P-3 at 308 nm and a broad, structured band at 370 nm are assigned as n(z)-n(y) and pi-pi transitions on the basis of CASSCF and CASPT2 calculations. The low intensity, long wavelength band of 3PN tailing to 500 nm is assigned as a mixture of pi-n(x), n(x)-pi* transitions. The decay of (PN)-P-1 in pentane was measured as a function of temperature to obtain observed rate constants of disappearance, k(obs). The rate constant k(obs) is equal to k(R) + k(ISC), where k(R) is the absolute rate constant of rearrangement of (PN)-P-1 to benzazirine and k(ISC) is the absolute rate constant of intersystem crossing to the lower energy triplet state ((PN)-P-3). When it is assumed that k(ISC) is independent of temperature, k(obs) can be dissected, and absolute values of k(R) and k(ISC) can be deduced. For (PN)-P-1, k(ISC) = 3.2 +/-0.3 x 10(6) s(-1) and the Arrhenius parameters for k(R) are found to be E-a = 5.6 +/- 0.3 kcal mol(-1) and A = 10(13.1+/-0.3) s(-1).