It has been established by continuous-flow studies in conjunction with EPR spectroscopy that the aminophosphonic acids 1–4 accelerate significantly the Fenton reaction between Fe^II and H₂O₂ in aqueous solution via complexation of the metal ion (with values of the rate constant k for the generation of the hydroxyl radical up to 2 × 10⁵ dm³ mol^–1 s^–1 at room temperature). To a certain extent this behaviour parallels that of EDTA and some structurally-related aminocarboxylic acids. It is also shown that the N-oxides of the aminophosphonic acids 1–3 react readily with the hydroxyl radical to give long-lived nitroxides viaβ-scission of first-formed carbon-centred radicals.

Neither of these findings is believed to correspond to the major chemistry which underlies the efficacy of these ligands as peroxide stabilizers. It is suggested instead that the crucial role of these compounds depends upon their ability to stabilize the higher valence state of iron, and hence not only to encourage oxidation of Fe^II by O₂˙^– and H₂O₂ but also to prevent effective reduction of Fe^III by O₂˙^–, HO₂˙ and H₂O₂. However, radical scavenging by N-oxides may be a secondary, contributory factor in this stabilizing function, especially in peroxide systems when the sequestrant is added before storage, when slow N-oxidation is to be expected.