Electrospray ionization of dilute aqueous solutions of copper(II) chloride-containing traces of pyridine (py) as well as ammonia permits the generation of the gaseous ions (py)2Cu+ and (py)2CuCl+, of which the latter is a formal copper(II) compound, whereas the former contains copper(I). Collision-induced dissociation of the mass-selected ions in an ion-trap mass spectrometer (IT-MS) leads to a loss of pyridine from (py)2Cu+, whereas an expulsion of atomic chlorine largely prevails for (py)2CuCl+. Theoretical studies using density functional theory predict a bond dissociation energy (BDE) of BDE[(py)2Cu+ -Cl] = 125 kJ mol−1, whereas the pyridine ligand is bound significantly stronger, i.e. BDE[(py)CuCl+ -py] = 194 kJ mol−1 and BDE[(py)Cu+ -py] = 242 kJ mol−1. The results are discussed with regard to the influence of the solvation on the stability of the CuI/CuII redox couple. Copyright © 2010 John Wiley & Sons, Ltd.