The reaction of the CH3 radical with HO2 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) CH4 + O-2 ((3)Sigma (-)(g)) and (b) CH4 + O-2 ((1)Delta) by direct H abstraction and (c) CH3O + OH and (d) CH2O + H2O by an association/decomposition mechanism via CH3OOH. 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 x 10(-16)T(1.25) exp(828/T) for 300-800 K, k(a) = 3.02 x 10(-21)T(2.83) exp(1877/T) for 800-3000 K, and k(c) = 2.97 x 10(-10)T(-0.24) exp(182/T) for 300-1000 K and 1.02 x 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 (CH3. . . HO2), which lies 1.9 kcal/mol below the reactants with a 1.2 kcal/mol barrier leading to the formation of the CH4 + O-2 ((3)Sigma (-)(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 CH3OOH agrees closely with the available experimental data using the heats of formation of CH3O (5.4 +/- 0.5 kcal/mol) and CH3OOH (-29.0 +/-1.0 kcal/mol) calculated with the isodesmic method at 0 K.