High level ab initio calculations are employed to investigate the excess electron attachment to the hydrated hydrohalogen acids. The excess electron leads to the dissociation of hydrogen halide acids, which results in the release of a hydrogen radical. Neutral HCl, HBr, and HI are dissociated by tetrahydration. Upon binding an excess electron, these hydrated hydrohalogen acids show that (i) the H-X bond strength weakens with redshifted H-X stretching frequencies, (ii) HX can have a bound-electron state, a dissociated structure, or a zwitter-ionic structure, and (iii) HClHBr is dissociated by tri/mono-hydration, while HI is dissociated even without hydration. This dissociation is in contrast to the case of electron attachment to hydrated hydrogen fluoric acids for which HF is not dissociated by more than ten water molecules.