Rationale: Diaryliodonium salts are useful electrophilic reagents in organic chemistry, finding extensive applications in arylations and photo-induced polymerizations. However, the comprehensive mechanistic investigations, particularly concerning the mass spectrometric behaviors of diaryliodonium salts, are relatively scarce in the literatures. Methods: Diaryliodonium salts could be readily ionized in ESI-MS to give [Ar 1-I +-Ar 2], and the high-resolution ESI-MS/MS experiments were conducted to investigate their gas-phase chemical reactions. Results: Investigations on ESI-MS/MS of [Ar 1-I +-Ar 2] revealed two major fragmentation patterns: 1) Reductive elimination resulting the diaryl coupling product ion [Ar 1-Ar 2] +• by the loss of I. 2) Generating aryl cations [Ar 1] + or [Ar 2] + through cleavage of the C–I bonds. We unrevealed that the introduction of NO 2 into Ar 2 of [Ar 1-I +-Ar 2] could lead to an unexpected fragmentation ion [Ar 1O] + in MS/MS, arising from an O-atom transfer process from NO 2 to Ar 1. Particularly, when NO 2 was ortho-positioned to the iodine in Ar 2, the [Ar 1O] + sometimes exhibited dominant behavior. Conclusions: Comprehensive ESI-MS/MS studies and theoretical calculations provided strong support for the O-atom transfer mechanistic pathway: [Ar 1-I +-( o-NO 2-Ar 2)] initially underwent a Smiles rearrangement to the intermediate [Ar 1-O-( o-NO-Ar 2I)] +, which subsequently dissociated to [Ar 1O] + or [ o-NO-Ar 2I] +•. Herein, we proposed an unexpected ” ortho-effect” in the gas-phase fragmentation reaction of [Ar 1-I +-( o-NO 2-Ar 2)], in which the crucial determinant factor for the aryl migration was identified as the Smiles rearrangement reaction.