This letter revisits critical intermediates and transition states of the C2H3 + O-2 reaction. To obtain their accurate relative energies, ab initio calculations are performed using sophisticated single and multireference theoretical methods with various basis sets. The energy difference between two crucial transition states, for ring opening in dioxiranylmethyl radical and its isomerization to C2H3OO, is calculated as similar to 2 kcal/mol both at multireference MRCI and at single-reference CCSD(T) levels extrapolated to the complete basis set limit. The deviation from the earlier G2M(RCC,MP2) value (similar to 7 kcal/mol) is caused by a deficiency of the 6-311+G-(3df,2p) basis set as compared to correlation-consistent Dunning's basis sets.