Laser flash photolysis (Nd:YAG laser, 355 nm, 35 mJ, 150 ps) of dimethyldiazirine and dimethyldiazirine-d(6) produces dimethylcarbene (DMC) and dimethylcarbene-d(6) (DMC-d(6)), respectively. The carbenes were trapped with pyridine to form ylides which absorb around 364 nm. It was possible to resolve the growth of the ylides as a function of pyridine concentration in Freon-113, alpha,alpha,alpha-trifluoromethylbenzene, and perfluorohexane as a function of temperature. The observed rate constant (k(obs)) of ylide formation was linearly dependent on the concentration of pyridine in all solvents and at all temperatures. From plots of k(obs) versus [pyridine] it was possible to extract values of k(pyr) (the absolute rate constant of reaction of the carbene with pyridine) and tau, the carbene lifetime in the absence of pyridine, and their associated Arrhenius parameters. In Freon-113 and alpha,alpha,alpha trifluoromethylbenzene the carbenes decay both by rearrangement and by reaction with solvent. In perfluorohexane the carbene decay appears to be predominantly unimolecular. The experimental results are compared with ab initio molecular orbital calculations. The experimentally determined barrier to disappearance of DMC in perfluorohexane (2.56 +/- 0.05 kcal/mol) is much smaller than that calculated (7.4 +/- 2 kcal/mol) using ab initio molecular orbital theory. The Arrhenius parameters and isotope effects indicate that the rearrangement of DMC in perfluorohexane has a large component of quantum mechanical tunneling. The activation energy for the disappearance of DMC-d(6) in perfluorohexane (5.63 +/- 0.03 kcal/mol) is consistent with calculations which indicate that QMT makes only a minor contribution to the deuterated system under the conditions of this study.