Recently, the chloronium ion H 2 Cl + was detected for the first time in the interstellar medium (ISM) by the Herschel Space Observatory using the Heterodyne Instrument for the far-Infrared (HIFI) . The dominant ionization state of each element in diffuse interstellar gas clouds is determined by its ionization potential. Chlorine has a slightly lower ionization potential than that of a hydrogen molecule, and is therefore mostly ionized individually in the form of Cl + . The latter ion can react with the dominant constituent of the interstellar medium H 2 to form the ion HCl + which, in turn, reacts with hydrogen to form the chloronium ion H 2 Cl + . Chemical modeling suggests that the chloronium ion could be very abundant around the diffuse ISM . However, prior to the launch of Herschel, H 35 Cl and H 37 Cl were the only Cl-bearing molecules detected in and around the ISM . This can be explained by the fact that the H 2 Cl + ion transforms into hydrogen chloride, HCl by dissociative recombination or by proton transfer to carbon monoxide CO. In addition to the detection by Herschel, another study reported by Neufeld et al , also detected absorption and emission of H 2 Cl + from two interstellar cloud sources. The chemical reactions that give rise to the deuterated forms HDCl + and D 2 Cl + are similar to those giving the chloronium ion, H 2 Cl + , with the same orders of magnitude of the reaction rate coefficient. It follows that, like the chloronium ion, its deuterated isotopologues should be abundant in the ISM. Surprisingly, to date, the existence of these deuterated isotopologues (HDCl + and D 2 Cl + ) has not been observed in the ISM.