FT-ICR (Fourier Transform Ion Cyclotron Resonance) experiments showed that the reaction of Cl2H+ with H2 (rate coefficient 6.3±1.0 cm3 s−1 molecule−1 at 298 K) yields ClH2+ ions, identified as protonated hydrochloric acid by CAD (Collisionally Activated Decomposition) mass spectrometry. Calculations performed at the B3LYP and CCSD(T) levels of theory show that the reaction involves insertion of the terminal Cl atom of Cl2H+ into the H–H bond, which accounts for the experimental observation that ClD2+ but not ClHD+ is formed from the reaction of Cl2H+ with D2. The same Cl+ insertion mechanism characterizes the reaction of Cl2H+ with CH4, whose product was identified as protonated methyl chloride, CH3ClH+, by CAD mass spectrometry.