This thesis deals with experimental studies of electron transfer reactions between oppositely charged ions
(mutual neutralisation). These were performed at the unique double electrostatic ion storage ring DESIREE at
Stockholm University, which was put into full operation in 2017. In the first two published articles of this thesis,
two atmospheric collision systems are treated, namely O+/O− and N+/O−. The aim was to reproduce previous
published results from a single-pass (non-stored) merged ion beams setup in UCLouvain (Belgium) and thus
provide a measure of DESIREE’s capacity and resolution. In addition, the effects of metastable ions were
investigated with the support of theoretical calculations. The third published paper of this thesis deals with
collisions between I+ and I− (iodine ions), a process relevant to electric thrusters for new spacecraft. The results
are compared with theoretical calculations in order to provide an understanding of how the reaction takes place.
Preliminary results on electron transfer reactions between diatomic molecules and atoms are presented.