The isomerization of singlet methylcarbene to ethylene has been studied using ab initio quantum mechanical methods, resulting in high-level theoretical predictions of the transition-state structure and isomerization barrier height. Basis sets as large as triple-zeta plus double polarization plus carbon atom f and hydrogen atom d functions [TZ2P(f,d)] have been used with the self-consistent-field (SCF) configuration interaction, including all single and double excitations (CISD), and coupled cluster, including all single and double substitution (CCSD) methods, as well as CCSD with the effects of connected triple excitations added perturbatively [CCSD(T)]. The geometries for ground-state singlet methylcarbene and the transition state are both of C-1 symmetry. The classical barrier is predicted to be 2.0 kcal mol(-1), while the activation energy (Delta E(0)) is 1.2 kcal mol(-1) at the TZ2P(f,d) CCSD(T) level at 0 K; and the free energy barrier is Delta G degrees = 1.5 kcal mol(-1) at 298 K. The activation energy is 0.3 kcal mol(-1) higher for fully deuterated singlet methylcarbene. Therefore singlet methylcarbene appears to be a true intermediate, consistent with some experimental deductions.