Laser flash photolysis (LFP) (XeCl, 308 nm, 17 ns) of phenyldiazomethane and pentafluorophenyldiazomethane releases phenylcarbene (PC) and pentafluorophenylcarbene (PFPC), respectively. In acetonitrile solvent the carbenes react rapidly to form nitrile ylides which have convenient absorption maxima for optical detection (lambda(max)=350 nm). Phenylcarbene and pentafluorophenylcarbene each react with acetonitrile with an absolute rate constant of 2.4x10(6) M-1 s(-1) in CF2ClCFCl2 (Freon-113) at ambient temperature. The lifetimes of spin-equilibrated PC and PFPC are 190 and 500 ns in Freon-113, respectively. and the lifetime of each carbene is deduced to be 22 ns in neat acetonitrile. LFP of phenyl and pentafluorophenyldiazomethane in the presence of pyridine leads to the expected pyridine ylide. The observed absolute rate constants of reaction of spin equilibrated PC and PFPC with pyridine are 1.9 x 10(7) and 5.1 x 10(7) M-1 s(-1), respectively, in Freon-113 at ambient temperature. From this data, one can deduce singlet-triplet splittings (Delta G(ST), 298 K) of 2.3 and 3.1 kcal/mol for PC and PFPC, respectively, which is the difference in energy between the triplet minimum and the point of intersection of the singlet and triplet carbene plus pyridine surfaces. LFP of phenyldiazomethane and pentafluorophenyldiazomethane in acetonitrile containing carbene scavengers (e.g., alcohols, alkenes, and silanes) gives reduced yields of nitrile ylides. Analysis of the yield of ylide as a function of quencher by the Stern-Volmer method gives k(Q) tau values of carbene quenching. Low-temperature (77 K) photolysis of pentafluorophenyldiazomethane generates the persistent EPR spectrum of triplet pentafluorophenylcarbene which is the ground state of this carbene. However, photolysis of pentafluorophenyldiazomethane at ambient temperature generates stable reaction products derived from capture of singlet pentafluorophenylcarbene. Thus the properties of pentafluorophenylcarbene are remarkably similar to those of phenylcarbene. Each carbene has a triplet ground state but reacts in solution at ambient temperature through a low-lying excited singlet state. The lack of a fluorine substituent effect on the behavior of phenylcarbene is compared to that in singlet phenylnitrene.