Three inelastic processes that can occur during collisions of high-velocity gas-phase ions with gas molecules are considered. These are: charge exchange (sometimes called electron transfer), in which, typically, one or more electrons from the gas are transferred to the ion; charge stripping, in which one or more free electrons leave the ion; and excitation, in which the ion and/or the gas become excited but neither changes its charge state. Deductions that can be made concerning the states populated in these processes and the critical energies involved from measurements of the translational energy loss (or gain) of the fast-moving product are outlined. Novel equipment designed to improve the attainable energy resolution in the third processes is described. For brevity, the discussion is restricted to consideration of positively charged ions.