Translational-energy spectra for state-selective single-electron capture by Ar2+ from atomic (He, Ne, Ar, Kr, Xe) and molecular (N2, O2, NO, N2O, NH3, CO2, CH4, C2H6, 1-C4H8, C6H6) target gases are recorded at 6 keV impact energy using a reversed-geometry double-focusing mass spectrometer. Spectra indicate the presence of the 3P, 1D and higher excited states of Ar2+, the populations of which are controlled by varying the ionising electron energy Ee. For Ar2+-He at Ee=70 eV, capture from the first excited 1D state of Ar2+ into the ground state Ar+ is most intense, whereas at Ee=43 eV, ground state to ground state capture dominates. For Ar2+ -rare-gas systems, relative cross sections are discussed in terms of calculated reaction windows. For molecular targets, dissociation is evident in many cases. Broad peaks and long tails in the translational energy spectra indicate reaction channels involving closely spaced energy levels. An additional peak due to the capture of delocalised electrons is observed for unsaturated hydrocarbons which has not been previously reported.