Ab initio methods have been employed to study the decomposition pathways of diazoethane and methyldiazirene as two possible precursors for the formation of methylcarbene. Geometries have been optimized at the SCF, MP2, and CISD levels of theory using 6-31G*, DZP, and TZP basis sets. Final energies were obtained at the TZP CCSD(T)//TZP CISD+ZPVE(DZP CISD) level. Vibrational frequencies were evaluated up to the DZP CISD level. Two possible mechanisms were considered : (a) simultaneous breaking of the C-N bond and migration of a methyl hydrogen to form ethylene and N-2, and (b) stepwise formation of methylcarbene and N-2 followed by rearrangement to ethylene and N-2. For both starting materials only the stepwise pathway was found. The activation barriers for the decomposition of diazoethane and methyldiazirene are 26.9 and 30.1 kcal/mol at our highest level, respectively. The barrier for the reverse association from methylcarbene + N-2 to diazoethane is only 3.3 kcal/mol; this barrier is 11.7 kcal/mol for methyldiazirene. There is no indication that surface crossings with the first excited states (generally >60 kcal mol(-1)) may occur. These results are in excellent agreement with experimental findings, and they nicely explain the difficulties involved in the isolation of methylcarbene, which lies on a small shoulder of the potential connecting the precursors and products. In view of further experimental work, methylcarbene should be more readily trapped using diazoethane as a precursor.