The performance of the B3LYP density functional theory calculations has been studied for the epoxidation reactions of ethylene, propene, and cis-and trans-2-butene with peroxyformic acid and of ethylene with dioxirane and dimethyldioxirane. The transition structures for the epoxidation of ethylene and propene with peroxyformic acid and of ethylene with dioxirane and dimethyldioxirane calculated at the B3LYP level as well as at the QCISD and CCSD levels are symmetrical with nearly identical C-O bond distances, whereas the MP2 calculations favor unsymmetrical transition structures. The geometrical parameters of the transition structures calculated using the B3LYP functional are close to those found at the QCISD and CCSD levels. While the activation barriers for the epoxidation reactions calculated at the B3LYP/6-31G* and B3LYP/6-31+G* levels are very close to the MP4SDTQ/6-31G*//MP2/6-31G* and MP2/6-31G*//MP2/6-31G* values, these activation energies are systematically lower (up to 5-6 kcal/mol) than the barrier heights calculated at such higher correlated levels as the QCISD(T)/6-31G*//QCISD/6-31G*, CCSD(T)/6-31G*//CCSD/6-31G*, and BD(T)/6-31G*//QCISD/6-31G*. The calculations on the epoxidation reactions of ethylene and propene with peroxyformic acid using the BH&HLYP functional also lead to symmetrical transition structures, but the calculated barriers are overestimated when compared with the QCISD(T) results. The activation barriers calculated for these epoxidation reactions at the QCISD(T)/6-31G*//B3LYP/6-31G* level are very close to those computed at the QCISD(T)/6-31C*//QCISD/6-31G* level.