The computational study of the high temperature CH3 + O-2 --> CH3OO --> CH2OOH --> CH2O + OH radical reaction was performed. The transition state for the CH3OO --> CH2OOH isomerization was generated using hybrid, exchange, Becke’s 88, Slater’s, and X alpha based density functional theory (DFT) methods. A large Gaussian 6-311+G(2d,2p) type basis set was used. The computed activation barriers varied with the correlation level included in the DFT methods, as well as with the nature of the electron density approximation. The best agreement between the computed energies and geometries was obtained with the hybrid, and in some cases with the gradient-corrected, DFT methods.