Laser flash photolysis (LFP) of perfluorophenyl azide and perfluoro-4-biphenyl azide produces the corresponding singlet nitrenes which were detected by their transient absorptions at 330 and 350 nm, respectively. The absolute rate constants of the fundamental processes that consume the singlet nitrenes (intersystem crossing, k(isc), rearrangement, k(R); reaction with pyridine, k(pyr)) were determined by monitoring the decay of the singlet nitrene and by the growth of its reaction products (ketenimine, triplet nitrene, or pyridine ylide). In the case of singlet 4-pefluorobiphenylnitrene in CH2Cl2 k(isc) = (2.2 +/- 0.1) x 10(6) s(-1), k(R) = 10(13.2+/-0.2) exp[-(9400 +/- 400)/RT] s(-1), and k(pyr) 10(9.06+/-0.15) exp[-(2400 +/- 200)/RT] M(-1) s(-1). In the case of singlet perfluorophenylnitrene in CH2Cl2 k(isc) = (1.05 +/- 0.05) x 10(7) s(-1), k(R) = 10(13.8+/-0.3) exp[-(8800 +/- 400)/RT] s(-1), and k(pyr) = 10(9.00+/-0.13) exp[-(1600 +/- 160)RT] M(-1) s(-1).