Unimolecular and collision-induced fragmentations of dialkyl peroxide/Fe+ complexes, ROOR’/Fe+ (R, R’ = methyl, ethyl, isopropyl, tert-butyl), were examined by means of tandem mass spectrometry. In the initial reaction step, iron bis(alkoxide) ions (RO)Fe(OR’)(+) are generated by insertion of the metal cation into the weak O-O bond of the peroxide. Depending on the nature of R and R’, five major reaction channels have been observed, corresponding to the generation of a methyl radical, methane, and water. Kinetic isotope effects are determined and interpreted in terms of rate-determining steps of the multistep processes. The observed reactivity can be described as a result of the balance of the energetics of beta-hydrogen versus beta-methyl shifts together with product stabilities. A scheme is presented, which correlates the unimolecular reactivity of ROOR’/Fe+ complexes with the substitution pattern in the alpha- and beta-positions of the peroxidic alkyl groups.