(i) Examination of open-shell dialkyl peroxide and alkyl hydroperoxide cation-radicals permits detailed insight in their peroxidic structure and the mechanisms of their unimolecular decomposition reactions. These processes can be described in terms of a general reactivity scheme and proceed via proton- or hydrogen-bridged species and ion/dipole complexes as intermediates.

(ii) By using a series of different neutralization-reionization techniques, evidence for the existence of reactive neutral molecules, e.g. several [C,H3,O2] radicals, is provided. In combination with high-level ab initio calculations, the first experimental evidence for the predicted existence of neutral water oxide, H2OO, is provided. Further, a new method will be presented, which allows to investigate the reactivity of neutral molecules in a mass spectrometer and neutral alkoxy radicals and peroxides serve as examples. Typical processes involve the a-scission, Barton-type hydrogen migrations, and O-O bond cleavage in neutral peroxides.

(iii) Closed-shell peroxidic species, i.e. protonated peroxides and hydroperoxy anions, are examined by mass spectrometric experiments as well as theoretical calculations. In these chapters the mechanistic course of the unimolecular decay of protonated peroxides as well as the oxenoid character of hydroperoxy anions are thoroughly discussed.

(iv) Ion/molecule reactions of „bare“ and ligated transition-metal cations with peroxides give insight into the conditions, which must be fulfilled for a formation of MO+ cations. These are highly potent oxidants, if M is a late transition metal. The dependence of reactivity on the location of the metal in the periodic table, on the internal energy of peroxide/M+ complexes, and on the structure of the peroxides will be discussed.