The geometries, energies, and vibrational frequencies of the reactants, transition states, intermediates, and products of the reaction of ethyl radical with the oxygen molecule have been examined using density functional theory (DFT). Rather different theoretical predictions are obtained from the BLYP, B3LYP, and BHLYP methods. Comparisons with experimental deductions and high-level coupled cluster results suggest that the B3LYP method is superior for the C2H5+O-2 problem. Using the B3LYP method with a triple-zeta plus double-polarization plus f function (TZ2Pf) basis set, a transition state between the ethylperoxy radical and products is discovered which lies 3.3 kcal mol(-1) below reactants. This transition-state energy is consistent with the observed high yields of ethylene in the high-temperature reaction and is in good agreement with the height of the barrier estimated via modeling of the experimental kinetic data. However, this transition state (TS1) corresponds not to the internal proton transfer leading to the hydroperoxyethyl radical C2H4OOH but to the concerted elimination of ethylene. For the reverse reaction C2H4+HO2-->C2H4OOH, the TZ2Pf UB3LYP classical barrier is 11.2 kcal mol(-1).