The Cl-atom-initiated photooxidation mechanism of CH3Br has been investigated in chamber experiments conducted under conditions applicable to the lower atmosphere. Major carbon-containing products obtained in the presence of O-2 and NO, as determined by FTIR absorption spectroscopy, are CH2O and CO. In addition, HC(O)Br is observed in the absence of NO. At low temperature, in the presence of NO, CH2BrO2NO2 was also identified as a reaction product. The results are used to determine the fate of the alkoxy radical, CH2BrO. It is found that unimolecular decomposition of CH2BrO to CH2O + Pr is the dominant process in 1 atm of O-2 at all temperatures studied (between 228 and 298 K), while reaction with O-2, CH2BrO + O-2 --> HC(O)Br + HO2, and the three-centered elimination of HBr, CH2BrO --> HCO + HBr, are found to be minor pathways under these conditions (less than 5% each). Assumption of a rate constant of 6 x 10(-14) cm(3) molecule(-1) s(-1) for the reaction of CH2BrO with O-2 (DeMore, W. B., et al. NASA JPL Publ. 1994, No. 94-26) provides a lower limit to the rate of Br-atom elimination of 3 x 10(7) s(-1) at 228 K, 1 atm of pressure.