Dimesitylcarbene (2a) was shown to decay by undergoing dimerization and to have a half-life of 160 ms, some 5 orders of magnitude longer-lived than diphenylcarbene. Didurylcarbene (2b) was twice as long-lived as 2a, while decamethyldiphenylcarbene (2c) decayed mainly unimolecularly by abstracting Il from the o-methyl group and became shorter-lived than 2b. 2,4,6-Tri(tert-butyl)diphenylcarbene (14) decayed unimolecularly almost exclusively by abstracting H from the o-tert-butyl group and showed the lifetime of 125 mu s. The Arrhenius plot for H and D abstraction of 2c and 14 gave data consistent with a classical atom-transfer reaction, although the kinetic parameters are very different between the two systems.