Methylcarbene, dimethylcarbene, chloromethylcarbene, and tert-butylmethylcarbene were studied at the HF/6-31G*, MP2/6-31G*, B3LYP/6-31G*, MP2/6-31+G(2d,p), QCISD(T)/6-31G*//MP2/6-31G*, and QCISD-(T)/6-31+G(2d,p) levels. Minimum energy geometries of the singlet and triplet states of the carbenes were calculated. The barriers to 1,2-hydrogen migration for CH3C-X-->CH2=CHX were calculated and found to increase for X = Cl > CH3 > C(CH3)(3) > H. Isodesmic calculations indicate that the effect of the bystander group is to stabilize differentially the carbene reactant, thereby increasing the barrier to rearrangement for X = Cl > CH3 > C(CH3)(3) > H.