The mechanism of the reaction of BrO + CH2O has been examined using density functional and quadratic configuration interaction molecular orbital methods. The most favorable pathway is found to be the abstraction of hydrogen from CH2O by BrO radicals. The activation energy barrier for the process is estimated as 2.9 +/- 3 kcal mol(-1). Results from discharge flow/mass spectroscopy experiments support the finding that the major products of the reaction are HOBr and HCO radicals. The rate of the BrO + CH2O reaction has been estimated by using transition state theory.