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.