Abstract High-resolution double-focusing mass spectrometry has been used to investigate the photofragmentation of Ar n CO + 2 clusters produced by electron impact ionization of the neutral cluster beam formed when a 300:1 mixture of Ar and CO 2 is expanded through a pulsed valve. The single line (514 nm) output from an argon ion laser is aligned coaxially with the ion beam in the second field free region of the mass spectrometer and fragments are analysed using the MIKES technique. Product branching ratios have been measured for n in the range 1–10. Small clusters show evidence of a dominant charge transfer reaction producing Ar + and Ar + 2 photoproducts. As the clusters grow in size, the charge has a tendency to remain with the CO + 2 containing moiety and excess energy is dissipated via monomer Ar evaporation. This effect suggests that the excited state takes the form Ar +·Ar n−1 ·CO 2, and that the solvation energy of Ar + does not parallel that of CO + 2 as the clusters increase in size.