The mechanism of the CH3O. + O-2 reaction in the gas phase leading to CH2O + HO2. was studied by using high-level quantum mechanical electronic structure calculations, The CASSCF method with the 6-311G-(d,p) basis set was employed for geometry optimization of 15 stationary points an the ground-state potential energy reaction surface and computing their harmonic vibrational frequencies. These stationary points were confirmed by subsequent geometry optimizations and vibrational frequencies calculations by using the CISD and QCISD methods with the 6-31G(d) and 6-311G(d,p) basis sets. Relative energies were calculated at the CCSD(T) level of theory with extended basis sets up to cc-pVTZ at the CASSCF/6-311G(d,p)-optimized geometries. In contrast to a recent theoretical study predicting an addition/elimination mechanism forming the trioxy radical CH3OOO. as intermediate, the oxidation of CH3O. by O-2 is found to occur by a direct H atom transfer mechanism through a ringlike transition structure of C-s symmetry. This transition structure shows an intermolecular noncovalent O ... O bonding interaction, which lowers its potential energy with respect to that of a noncyclic transition structure by about 8 kcal/mol. The 1,4 H atom transfer in CH3OOO. is not accompanied by HO2. elimination but leads to the trioxomethyl radical (CH2OOOH)-C-. via a puckered ringlike transition structure, lying 50.6 kcal/mol above the energy of the reactants. The direct H atom transfer pathway is predicted to occur with an Arrhenius activation energy of 2.8 kcal/mol and a preexponential factor of 3.5733 x 10(-14) molecule cm(3) s(-1) at 298 K. Inclusion of quantum mechanical tunneling correction to the rate constant computed with these parameters leads to a rate constant of 2.7 x 10(-15) molecule(-1) cm(3) s(-1) at 298 K, in good agreement with the experimental value of 1.9 x 10(-15) molecule(-1) cm(3) s(-1).