The structures, vibrational frequencies, and rotational constants have been determined using ab initio methods of isomers of BrClO3. The BrOCl(O)O structural form is found to be the most stable. This species could be formed from the three-body reactions involving BrO + OClO + M --> BrOCl(O)O + M and Br + ClO3 + M --> BrOCl(O)O + M. The heats of reaction for these processes are estimated to be -8.2 and -24.4 kcal mol(-1), respectively.