High level ab initio electronic structure calculations at the coupled cluster level with a correction for triples extrapolaled to the complete basis set limit have been made for the thermodynamics of the BrBrO2, IIO2, ClBrO2, ClIO2, and BrIO2 isomers, as well as various molecules involved in the bond dissociation processes. Of the BrBrO2 isomers, BrOOBr is predicted to be the most stable by 8.5 and 9.3 kcal/mol compared to BrBrO2 and BrOBrO at 298 K, respectively. The weakest bond in BrOOBr is the O-Br bond with a bond dissociation energy (BDF) of 15.9 kcal/mol, and in BrBrO2, it is the Br-Br bond of 19.1 kcal/mol. The smallest BDE in BrOBrO is for the central O-Br bond with a BDE of 12.6 kcal/mol. Of the IIO2 isomers, IIO2 is predicted to be the most stable by 3.3, 9.4, and 28.9 kcal/mol compared to IOIO, IOOI, and OIIO at 298 K, respectively. The weakest bond in IIO2 is the I-I bond with a BDF of 22.2 kcal/mol. The smallest BDEs in IOIO and IOOI are the terminal O-I bonds with values of 19.0 and 5.2 kcal/mol, respectively.