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As a cheap but harmful additive in food industry, 1-phenylazo-2-naphthol (Sudan I) has attracted lots of attentions, which mainly focused on the determinations using the HPLC-ESI-MS/ MS techniques. However, the fragmentation mechanisms are generally scarce. In this paper, the fragmentation pathways of protonated Sudan I in gas-phase were investigated by a combination of electrospray ionization-tandem mass spectrometry and density functional theory (DFT) calculations. Initially, the cleavage of azo bond is triggered by ionizing proton migration from the oxygen atom of O-protonated Sudan I to adjacent N atom through a six-member ring transition state, leading to a single ion-neutral complex (INC-s). Subsequently, the direct decomposition of INC-s gives the naphtol nitrene ion at m/z 156. Meanwhile, INC-s may occur an intermolecular single electron transfer (SET) reaction via the crossing points (INC-mecp), affording the triple ion-neutral complex (INC-t) consisting of naphtol nitrene radical and aniline radical ion. Finally, followed by the decomposition of INC-t, the aniline radical ion at m/z 156 is formed. Further studies on Electrostatic potential map, Natural population analysis (NPA) and frontier molecular orbitals (FMOs) during the proposed mechanism are also found to be in good agreement with experimental observations. Taken together, our results presented here not only provide mechanistic information for better understanding the Sudan I, but also give insights into other related harmful additives in food industry.