[en] The selected ion flow drift tube has proven valuable for isotopic studies of gas-phase ion-molecule reactions. Both mechanistic and dynamical studies can be performed. Both aspects will be covered in this presentation. The reaction of O^- with N₂O to produce NO^- and NO will be used to show how a labeling study might yield information on reaction mechanism. This study was initiated to determine whether the mechanism proceeded by attack on the terminal or central N. Even the totally labeled experiment, however, failed to reveal the mechanism conclusively. Also to be presented is the effect of various types of energy on reactions involving isotopes. Examples will be shown of the influence of kinetic, rotational, and vibrational energy on both rate constants and branching ratios of ion-molecule reactions