Abstract: | A variety of double collision experiments, whereby fast species undergo collisional interactions in two distinct regions of a mass spectrometer, are described. These include two-stage charge reversal of negative ions, two-stage double electron transfer from targets to cations, neutralization-reionization experiments as well as delayed analysis of organic cations formed in a one-step charge reversal of anions. Experiments have been performed on a number of systems of current interest in gas-phase ion chemistry. It is concluded that autoelectron detachment of benzyl anions leads to benzyl radicals, whereas the collisionally induced electron detachment produces a mixture of benzyl and tropyl radicals. By contrast, electron detachment from [H3CNH]? is not a metastable process and occurs only after excitation to produce H3CNH˙ radicals, which do not rearrange into the thermodynamically more stable H2CNH2˙. It is shown that in the double electron transfer reactions H+ + Xe→H˙ + Xe+˙ and H˙ + Xe→H? + Xe+˙, excited states are produced. From double collision experiments on methyl formate ions, it is concluded that the non-decomposing ions have undergone rearrangement on the time-scale of 10 μs into the distonic isomer, documentclass{article}pagestyle{empty}begin{document}$ {rm H} - mathop {rm C}limits^ + ({rm OH}){rm O}mathop {rm C}limits^. {rm H}_2 $end{document} . Finally, it is shown that short-lived (<0.2 μs) [H2, C, N]+ ions generated by charge-reversal of [H2CN]? have the [H2CN]+ structure, whereas most of the long-lived (10 μs) ions have the [HCNH]+ structure. |