This chapter discusses the effects of spin‐changes in various gas‐phase reactions with the focus on reactions where spin‐changes actually accelerate the observed reactions or allow them thermodynamically. The major methods that were used for the investigations of the reactions discussed in the chapter are trapping techniques, guided‐ion beam methods and selected ion flow tube. Typical examples of a multiple state reactivity can be found in transition metal chemistry. The influence of the interplay of spin states along reaction coordinates is demonstrated on the reactivity of metals and their different compounds toward methane. Properties of transition metals can be tuned by addition of ligands and by changing their oxidation states. The simplest example is an addition of a hydride ligand to transition metal cations, which results in a change of their oxidation state and thus opens up the possibility of employing different spin states along the reaction coordinates.