Abstract: A review of the unimolecular reactions of even electron ions is presented.Both "onium" and carbenium ions are treated. The nomenclature and structureof the former is briefly touched. A comparison between the unimolecularreactions of even electron ions and those of their odd electron counterparts ismade where appropriate. Low energy (metastable ions) or high energy (colli-sionally activated) unimolecular reactions are reviewed. Besides, three types ofreactions which are typical and important either in the fragmentation of evenelectrons or in the odd electron ones, viz. the loss of functional groups inchemical ionization, McLafferty rearrangement and retro-Diels-Alder reactionare discussed.