Unimolecular and collision-induced fragmentations of dimethyl peroxide complexes (CH3OOCH3)M(+), with M(+) = Cr+, Mn+, Fe+, and Co+, were examined by means of tandem mass spectrometry and compared to results from bimolecular gas-phase reactions of the corresponding ''bare'' metal cations with dimethyl peroxide as studied by Fourier transform ion cyclotron resonance (FT-ICR) experiments. Bismethoxide metal ions, (CH3O)(2)M(+), are generated in the initial reaction step by insertion of the metal cation into the weak O-O bond of the peroxide. In the ion-beam apparatus these intermediates undergo a variety of processes involving beta-H shifts, intramolecular redox reactions, and radical losses, depending on the respective metal cations. In contrast, the insertion products (CH3O)(2)M(+) formed in the ion/molecule reactions of dimethyl peroxide with bare metal cations in the FT-ICR mass spectrometer decompose to complexes M(OCH3)(+) with concomitant loss of a methoxy radical, a process which is not observed in the unimolecular dissociation of metastable (CH3OOCH3)M(+) ions. The distinct differences between the unimolecular chemistry of dimethyl peroxide/M(+) complexes in the ion-beam apparatus and the bimolecular reactions of M(+) with dimethyl peroxide in the FT-ICR are explained in terms of internal energy effects, which result from the method of ion formation. This analysis is further supported by ligand-exchange reactions and collisional-activation experiments. In addition, analogies and differences of the gas-phase chemistry of dimethyl peroxide with M(+) as compared to metal-catalyzed decomposition of dialkyl peroxides in the condensed phase are discussed.