a b s t r a c t The high-temperature bromine chemistry was updated and the inhibition mechanisms involving HBr and Br 2 were re-examined. The thermochemistry of the bromine species was obtained using the Active Ther-mochemical Tables (ATcT) approach, resulting in improved data for, among others, Br, HBr, HOBr and BrO. Ab initio calculations were used to obtain rate coefficients for selected reactions of HBr and HOBr, and the hydrogen/bromine/oxygen reaction mechanism was updated. The resulting model was validated against selected experimental data from the literature and used to analyze the effect of HBr and Br 2 on laminar, premixed hydrogen flames. Our work shows that hydrogen bromide and molecular bromine act differ-ently as inhibitors in flames. For HBr, the reaction HBr + H H 2 + Br (R2) is rapidly equilibrated, deplet-ing HBr in favor of atomic Br, which is the major bromine species throughout the reaction zone. The chain-breaking steps are then H + Br + M ? HBr + M (R1), Br+HO 2 ? HBr + O 2 (R7), and Br + Br + M ? Br 2 + M (R8). In Br 2 -doped flames, the reaction Br 2 + H HBr + Br (R9) is far from equili-bration and serves to deplete H in the reaction zone by competing with H + O 2 ? O + OH. The inhibition is augmented by recombination of Br (R8). If the inlet Br 2 mole fraction exceeds about 20%, reactions (R8) and (R2) are both reversed, now acting to promote chain branching and increase the flame speed. Accord-ing to the present model, cycles involving HOBr are not important for generation or removal of chain car-riers in these flames.