This study investigates the 'decomposition of 2,2',4,4',5-pentabrorminated diphenyl ether (BDE99), a commonly detected pollutant in the environment. Debromination,channels yielding tetrabrominated diphenyl ethers and hydrogen abstracting aromatic bromine atom formations play significant roles in the reaction of BDE99 + in which the former absolutely predominates bimolecular reactions. Polybrominated dibenzo-p-dioxins (PBDDs) and polybrominated dibenzofurans (PBDFs) Can be produced during BDE99 pyrolysis, especially for PBDFs under inert conditions. The expected dominant pathways in a closed system are debromination products and PBDF formations. The bimolecular reaction with hydroxyl radical mainly leads to hydroxylated BDE99S rather than hydroxylated tetrabrominated diphenyl ethers. PBDDs are then generated from ortho-hydroxylated PBDEs. HO2 radical reactions rarely proceed. The total rate constants for the BDE99 reaction with hydrogen atoms and hydroxyl radicals exhibit positive dependence on temperature with values of 1.86 X 10(-14) and 5.24 x 10(-14) cm(3) molecule(-1) s(-1) at 298.15 K, respectively.