The structural and energetic properties of the HOBr --> HBrO transition state are examined using the single and doubles coupled-cluster method that includes a perturbational estimate of the effect of connected triple excitations [CCSD(T)]. The energy change for the isomerization reaction is best estimated to be 56.5 kcal mol(-1) endothermic, and the activation energy for the process is 75.0 kcal mol(-1).