Laser flash photolysis (308 nm, 20 ns, 150 mJ) of 4-azido-2,3,5,6-tetrafluorobenzamides, esters, and thioesters generates singlet nitrenes which can be intercepted with pyridine to produce strongly absorbing ylides. It was possible to resolve the rate of formation of the ylides as a function of pyridine concentration. This has led to direct measurements of the absolute rate constants of (a) the reaction of the nitrene with pyridine, (b) ring expansion of the nitrene to a ketenimine, and (c) singlet to triplet nitrene intersystem crossing.