Ultrafast photolysis of p-biphenylyldiazoethane (BDE) produces an excited state of the diazo compound in acetonitrile, cyclohexane, and methanol with lambda(max) = 490 nm and lifetimes of less than 300 fs. The decay of the diazo excited state correlates with the growth of singlet carbene absorption at 360 nm. The optical yields of diazo excited states produced by photolysis of p-biphenylyldiazomethane (BDM) and BDE are the same; however, the optical yield of singlet p-biphenylylmethylcarbene ((1)BpCMe) is 30-40% less than that of p-biphenylylcarbene ((1)BpCH) in all three solvents. The results are explained by rearrangement in the excited state (RIES) of BDE to form p-vinylbiphenyl (VB) in parallel with extrusion of nitrogen to form (1)BpCMe in reduced yield. This interpretation is consistent with product studies (ethanol-OD in cyclohexane) which indicate that there is an similar to 25% yield of VB that is formed by a mechanism that bypasses the relaxed singlet carbene. The decay of (1)BpCMe is biexponential, and that of (1)BpCH is monoexponential. This is attributed either to efficient relaxation of vibrationally excited (1)BpCMe by 1,2 migration of hydrogen to form VB (minor) or to the increased number of low-frequency vibrational modes provided by the methyl group (major). A methyl group retards the rate of intersystem crossing (ISC), relative to a hydrogen atom, and ISC is more rapid in nonpolar solvents. Reaction of (1)BpCMe with methanol is much faster than spin equilibration. Both the lifetime of (1)BpCMe and (1)BpCH are the same in cyclohexane and in cyclohexane-d(12). This demonstrates that spin equilibration is faster than reaction of either carbene with the solvent. The lifetimes of (1)BpCMe and (1)BpCMe-d(3) are the same in cyclohexane. This indicates that 1,2 hydrogen migration of (1)BpCMe to form VB is slower than spin equilibration in cyclohexane. In acetonitrile, however, the lifetime of (1)BpCMe-d(3) is 1.5 times longer than that of (1)BpCMe in the same solvent. Thus, in acetonitrile, where ISC is slow, the rate of 1,2 hydrogen shift of (1)BpCMe is competitive with ISC. In cyclohexene, the lifetime of (1)BpCH is shortened relative to that in cyclohexane. The lifetime of (1)BpCMe is the same in cyclohexene and cyclohexane. The data indicate that spin relaxation is slow relative to reaction of (1)BpCH with neat alkene but that spin relaxation is fast for (1)BpCMe relative to reaction with neat cyclohexene.