The stereochemistry of 1,2-H migration in ethylchlorocarbene (1) and chloromethylchlorocarbene (2) has been studied by ab initio methods. Geometries of the ground and transition states of a conformational equilibrium and the 1,2 rearrangement were optimized at the DFT (B3LYP) and MP2 levels of theory using 6-31G(D) and 6-311+G(D,P) basis sets. Final energies were obtained at the MP4/6-311+G(D,P)//MP2/6-311+G(D,P) level. It has been shown that the equilibrium between cis- and trans-conformers of 1 and 2 is shifted moderately toward the trans-conformer for carbene 1 and strongly toward the cis-conformer in the case of 2. The calculated barriers of rotation about the CC bond in carbene 1 (Delta G double dagger = 2.3 kcal mol(-1)) and 2 (5.3 kcal mol(-1)) are lower than the smallest predicted barriers of the 1,2-H shift (8.0 and 8.5 kcal mol(-1), respectively). In accordance with the Curtin-Hammett principle, kinetic control of stereochemistry of the rearrangement proceeding classically is realized. The predicted preferable formation of the Z-isomer of 1-chloropropene (3) and 1,2-dichloroethylene (4) is in good agreement with the experimental data obtained under conditions of the high-temperature thermolysis of the corresponding diazirines. Electronic factors influencing the relative stability of the cis- and trans-isomers of carbenes 1 and 2 and their transition states for 1,2-H migration are discussed.