The rate constant of the reaction(R1)OH+CH3O2→productshas been measured at 294K by simultaneous coupling of Laser Induced Fluorescence (LIF) and cw-Cavity Ring Down Spectroscopy (cw-CDRS) to laser photolysis. OH radicals were generated by 248nm photolysis of either H 2 O 2 or O 3 in the presence of H 2 O, CH 3 O 2 radicals were generated simultaneously by photolysis of CH 3 I and their absolute concentrations were obtained by cw-CRDS. OH decays were measured under excess CH 3 O 2 concentrations and a very fast rate constant of k 1 =(2.8±1.4)×10 −10 cm 3 s −1 was found independent of pressure at 50 and 100Torr helium.