Numerous transition metal ions and their complexes in their lower oxidation states (LmMn+) were found to have the oxidative features of the Fenton reagent, and, therefore, the mixtures of these metal compounds with H2O2 were named "Fenton-like" reagents. Using the Marcus theory and the experimental data in the literature, it is shown that in most cases the reaction of these metal complexes with H2O2 is unlikely to occur via an outer-sphere electron-transfer mechanism. It is suggested that the first step in this process is the formation of a transient complex LmM-H2O2n+, which may decompose to an .OH radical or a higher oxidation state of the metal, LmM(n + 2)+, or it may yield an organic free radical in the presence of organic substrates. Thus, the question whether free .OH radicals are being formed or not via the Fenton reaction depends on the relative rates of the decomposition reactions of the metal-peroxide complex and that of its reaction with organic substrates. Contradictory conclusions described from the study of different systems might only indicate that these relative rates are different in these systems.