High level ab initio and CASSCF calculations on the epoxidation of ethylene with dioxirane (DO) and dimethydioxirane (DMDO) have been carried out to distinguish between a symmetrical versus an unsymmetrical spiro orientation of the dioxirane in the transition structure for oxygen atom transfer. The optimized C-1 DO/ ethylene unsymmetrical spiro CASSCF(12,12)/6-31G(d) transition structure is a first-order saddle point that is 5.5 kcal/mol lower in energy than the corresponding constrained C-S symmetrical approach (a second-order saddle point) after correction for dynamic correlation [CASSCF(MP2)]. However, a single-point energy correction at the BD(T)/6-311 +G(d,p) level on these CASSCF geometries suggests that the C-1 TS is only 0.6 kcal/mol lower in energy than an symmetrical spiro TS. Both the BD(T) correction to the CAS(8,8)/6-31+G-(d,p) and RSPT2 energy correction on CAS(8,8)/6-31G(d) DO/ethylene structures slightly favors the C-S structures. BD(T)/6-311+G(d,p) single point calculations on the C-1 and C-S transition structures optimized at the CCSD(T)/6-31G(d) level slightly favor the unsymmetrical TS. These combined data suggest that the potential energy surface for the approach of dioxirane to the C=C of ethylene is very soft with the C-1 transition structure being slightly favored. For DMDO epoxidation, we conclude that the approach of DMDO to a symmetrically substituted alkene should result in an essentially symmetrical transition structure.