The reaction of the CH 3 radical with HO 2 has been investigated by means of ab initio molecular orbital theory and variational RRKM theory calculations. The reaction can take place by several product channels producing (a) CH 4 + O 2 ( 3 Σ g - ) and (b) CH 4 + O 2 ( 1 Δ) by direct H abstraction and (c) CH 3 O + OH and (d) CH 2 O + H 2 O by an association/decomposition mechanism via CH 3 OOH. The bimolecular reaction rate constants for the formation of these products have been calculated for the temperature range 300-3000 K and found to be pressure independent up to 50 atm. The Arrhenius equations for the two major channels a and c were found to be strongly curved; they can be represented by k a = 4.23 × 10 -16 T 1.25 exp(828/T) for 300-800 K, k a = 3.02 × 10 -21 T 2.83 exp(1877/T) for 800-3000 K, and k c = 2.97 × 10 -10 T -0.24 exp(182/T) for 300-1000 K and 1.02 × 10 -13 T 0.76 exp(1195/T) for 1000-3000 K, in units of cm 3 molecule -1 s -1 . In the abstraction channel a, the effect of multiple reflections above its van der Waals complex (CH 3 ⋯HO 2 ), which lies 1.9 kcal/mol below the reactants with a 1.2 kcal/mol barrier leading to the formation of the CH 4 + O 2 ( 3 Σ g - ) products, was found to be quite significant at low temperatures (T < 300 K). In addition, the predicted rate constant for the unimolecular decomposition of CH 3 OOH agrees closely with the available experimental data using the heats of formation of CH 3 O (5.4 ± 0.5 kcal/mol) and CH 3 OOH (-29.0 ± 1.0 kcal/mol) calculated with the isodesmic method at 0 K.