Protonated hydrogen chloride or chloronium (H₂Cl⁺) was identified in 2010 by Lis et al. with the Herschel Space Observatory. It was observed in the massive hot core NGC 6334I in the Cat's Paw nebula, as well as toward Sgr B2(S). A follow-up study by Neufeld et al. published in 2012 also detected H₂Cl⁺ toward Sgr A, W31C, two sources in the Orion Molecular Cloud 1 (the Orion Bar and Orion South), and the young massive star AFGL 2591.

Chloronium is isoelectronic with hydrogen sulfide (H₂S) and has a similar structure. The near 90° bonds in both compounds occur because the second X–H bond is a covalent bond between H and a singly occupied 3p orbital that is perpendicular to the bond axis in HCl⁺ and SH (which are both ²Π states).