CH3O((XE)-E-2) radicals have been detected by laser-induced fluorescence (LIF) for the first time as a product of the reaction between CH3 and O-3. CH3 radicals were generated by excimer-laser photolysis of CH3Br at 193 nm in the presence of O-3 at room temperature and 100 Torr of He. The rate constant for reaction 1a (CH3 + O-3 --> CH3O + O-2) has been determined by monitoring the CH3O build-up as a function of reaction time and k(1a) was found to be (9.68 +/- 1.10) x 10(-14) cm(3) molecule(-1) s(-1). The rate coefficient of the global reaction 1 (CH3 + O-3 --> products) has been determined by the numerical analysis of the CH3O temporal profiles, yielding a value of k(1) = (2.2 +/- 0.3) x 10(-12) cm(3) molecule(-1) s(-1). Reaction of CH3 with NO2 has also been studied under the same experimental conditions as reaction 1 and has been used to calibrate the CH3O LIF signal. On the other hand, the rate constant k(2) for reaction 2 (CH3O + O-3 --> products) has been directly determined (k(2)(T = 298 K) = (2.53 +/- 0.75) x 10(-14) cm(3) molecule(-1) s(-1)) using the CH3ONO(lambda = 193 nm)/O-3/He system and was included in the chemical model used to describe the kinetics of CH3 in the presence of O-3. The branching ratio phi(CH3O) = k(1a)/k(1) obtained was found to be (0.044 +/- 0.013). This low yield of CH3O in reaction la can be explained not only by the formation of highly vibrationally excited CH3O and the subsequent prompt dissociation to produce CH2O + H but also by the direct formation of CH2O + HO2 in reaction 1.