A process for the selective oxidation of aromatic compounds into nitroso compounds based on a ternary system by simply mixing sodium decatungstate (NaDT)/Fe3O4/H2O2 in an aqueous solution has been designed. NaDT and Fe3O4 were well characterized via XRD, FTIR, TEM, XPS, etc. The catalytic selective oxidation performance of different systems (NaDT/Fe3O4/H2O2, NaDT/H2O2, etc.) was compared. The effects of different parameters (pH, H2O2, Fe3O4, and NaDT concentrations) are also investigated. In addition, this study is focused on the catalytic product analysis and catalytic mechanisms. The NaDT and Fe3O4 were successfully synthesized and the interaction between Fe3O4 and NaDT was found. Comparing the oxidation efficiency of sulfapyridine in the NaDT/H2O2 binary system, the efficiency increases, in NaDT/Fe3O4/H2O2 ternary system, by approximately 380 times. Nitroso-sulfapyridine (N-SPD) was shown to be the main product but hydroxy-sulfapyridine (SPD-OH) was produced, representing a maximum of 5 % SPD conversion, due to the presence of •OH radicals in the system. The oxidation efficiency and the selective conversion of sulfapyridine (SPD) to N-SPD reached up to 97 % and 81 %, respectively. The rate constant and the percentage of SPD disappearance reach a plateau when concentrations of NaDT, Fe3O4, and H2O2 increase, with limit concentrations of approximately 40 µM, 0.8 g L− 1, and 10.0 mM respectively. The obtained results demonstrated that the oxidation of SPD into N-SPD is efficient at pH lower than 5.0. The effectiveness of the system is based on a complex mechanism obtained through interactions between Fe3O4 and NaDT, confirmed by the FTIR and XPS measurements, on one side and between NaDT and H2O2 on the other side, as clearly demonstrated in cyclic voltammetry (CV). These interactions in the ternary system accelerate the redox cycle of NaDT and Fe3O4 in the oxidation reaction, and facilitate the transfer of electrons from SPD to H2O2, contributing to the efficiency of SPD oxidation into N-SPD.