The ab initio constant-pressure molecular dynamics method was applied to investigate the high-pressure chemistry of hydrogen-bended solid HBr. It is shown that the molecular solid becomes unstable against elimination of Hz at pressures beyond about 20 GPa. Our simulation suggests that the undercoordinated Pr atoms in H-deficient HBr art: not reactive under high pressure because of delocalization of the holes doped by H vacancies and the high diffusivity of H atoms. We also demonstrate that decompression process can reactivate undercoordinated Br atoms to form Br-2 Mechanisms to explain these instabilities and solid-state reactions are discussed.