The properties of HCl⁺ ions formed in the vibronic ground state ²Π_3/2 (″ = 0) ia the R(1) pump line of the f³Δ₂←¹Σ⁺ REMPI spectrum have been investigated. In the first experiment the rotational state distribution of these ions has been determined by photoexcitation into the A²Σ⁺(′ = 7) state, which can predissociate. 73% of the HCl⁺ ions are found in the N″ = 0 state, 23% in the N″ = 1 and 4% in the N″ = 2 state. This experiment also yields the spectroscopic parameters of the A²Σ⁺(′ = 7) state and information on the rotational dependence of the predissociation lifetimes of this state. The threshold for predissociation occurs between the N′ = 0, J′ = 1/2 and the N′ = 1, J′ = 1/2 state, leading to a new dissociation energy of the HCl⁺ ion of D₀ = 37536.5 cm⁻¹±5 cm⁻¹. In the second experiment these state-selected HCl⁺ ions are investigated in an ion–molecule reaction with carbon monoxide. In this reaction very efficient proton transfer leads to the formation of HCO⁺ with a rate constant close to the Langevin limit.