This thesis deals with experimental studies of electron transfer reactions between oppositely charged ions (cations and anions), in a process called mutual neutralisation. These investigations were performed at the double electrostatic ion storage ring DESIREE at Stockholm University, which was put into full operation in 2017. This unique apparatus consists of two cryogenic electrostatic rings where oppositely charged ion beams are stored and merged in a common section where the reactions of interest take place. The neutral products arising from the reactions are detected in coincidence using a sensitive 3D imaging detector. This approach allows the kinetic energy of the products to be measured, and the particular product channels to be identified, such that the branching ratio into the different competing sets of products can be determined. 

The reactions studied in this thesis take place in both natural plasmas, such as our own atmosphere, as well as industrial ones, such as ion propulsion engines and fusion reactors. For the first time, the final-state distribution of the products in a number of mutual neutralisation reactions involving molecular ions were determined, and the reaction dynamics were elucidated. 

The work presented in this thesis deals with the development of data analysis methods necessary for the imaging of coincidence products, as well as simulation methods required to interpret the experimental results. The thesis touches briefly the current theoretical models treating these reactions and their current recent development and results.