Substituent effects at the migration origin on the rate of rearrangement of several alkylchlorocarbenes have been studied at the B3LYP/6-311G**//B3LYP/6-31G* level. Methyl, halo, and phenyl groups are found to accelerate 1,2-H shift rearrangements in the order Ph > Me > F > Cl > H. Methyl groups are also found to accelerate both singlet and triplet 1,2-Ph shifts in benzylchloro- and benzylcarbenes. A direct comparison of the 1,2-H shift in methylchlorocarbene and the 1,2-Ph shift in benzylchlorocarbene shows that the inherent migratory preference of a phenyl group is greater than that of a hydrogen atom. Significant bystander substituent effects are found, as originally proposed by Nickon.