Abstract: Gas-phase ion/ion reactions can be highly efficient, fast, specific and informative, and can be readily implemented in mass spectrometry. They are particularly promising as means for converting an ion formed directly by an ionization technique into a wide array of other ion types that can generate additional information regarding an analyte of interest. The gas-phase ion/ion reactions can be divided into three categories: acid/base neutralization reactions, redox reactions and covalent chemistries. Acid-base neutralization reactions can be subdivided into three types: proton transfer reactions, metal ion transfer reactions and charge inversion reactions. In addition, redox reactions are accompanied by electron transfer, so it is also known as electron transfer reactions. Compared to former ion/ion reaction devices based on quadrupole mass spectrometry and time-of-flight mass spectrometer, linear ion trap is proved as a proper reactor for gas phase ion/ion reaction, as it has good ionic selection and enrichment function. In addition, a key advantage of the use of a linear ion trap as the reaction vessel is its well-known capability for executing MSⁿ experiments as a result of the “tandem-in-time” nature of the device. Nowadays gas-phase ion/ion reactions play an important role in study of organic chemical reaction mechanism, analysis of polypeptide structure, and ion chemistry. And they may play a greater role in chemical, biological and other fields in the future.