Bromine peroxide, mixed bromine and chlorine peroxides, and their isomers have been studied ab initio using both wave function and density functional theory methods. The performance of the methodologies in studying these species is analyzed. Optimized geometries, harmonic frequencies, relative energies, and isodesmic heats of formation at 0 and 298.15 K, have been calculated and used to discuss the form and relative stability of these species. The spontaneity of various possible decomposition channels for these species is also studied. Comparisons with previous ab initio calculations is presented and discrepancies are analyzed.