The rate coefficient, k 1 (298 K) = (5.0 ± 1.1) 10 - 1 1 cm 3 molecule - 1 s - 1 , for the reaction, Br + IO → I + BrO (1) was determined by monitoring the temporal profiles of IO in an excess of Br. The rate coefficient for the reverse reaction, I + BrO → IO + Br (-1), k - 1 (298 K) = (1.9 ± 1.1) 10 - 1 1 cm 3 molecule - 1 s - 1 , was determined by modeling IO temporal profiles, measured in the presence of BrO and O 3 , in which the regeneration of IO was observed. From these rate coefficients, the difference in the heats of formations of IO and BrO, Δ f H° (298 K) (BrO) - Δ f H° (298 K) (IO), was calculated to be 0.5 ± 0.6 kcal mol - 1 .