The rate const. for the reaction (1) CH3O + O -> products at 298 K has been detd. using 248 nm laser co-photolysis of CH3ONO/O3 mixts. for the generation of CH3O radicals and O atoms combined with LIF (laser-induced fluorescence) for time resolved detection of CH3O and product OH. The value of k1 is (2.5 +- 0.7) 10-11 cm3/s with a relative yield of OH of j = -D[OH]/D[CH3O] = 0.12-0.04+0.08. The results are shown to be consistent with a primary capture mechanism on an electronically adiabatic surface forming highly vibrationally excited CH3O2* and followed by rapid subsequent decompn. or isomerization, viz.: CH3O + O -> CH3O2* -> CH3 + O2 and CH3O2* -> CH2OOH.thermod. -> CH2O + OH. From modeling of the OH yield using RRKM theory the isomerization barrier between CH3O2 and CH2OOH is placed near 160 kJ/mol in good agreement with recent ab initio calcns. As a consequence the CH3 + O2 combustion reaction is predicted to proceed primarily via the low energy CH2O + OH channel, in contrast to most previous suggestions.