The ion/molecule reactions of the phenylarsandiyl radical cation C 6H 5As +, 1 +, with methyl-halides CH 3X, X = Cl, Br, I, have been investigated using Fourier transform ion cyclotron resonance (FT-ICR) spectrometry. The radical cation 1 + exhibits in its valence electron shell a vacant molecular orbital as well as a single occupied orbital and an orbital containing an electron pair. Consequently, reactivity of an electrophilic carbenoid and/or an electrophilic radical is expected. No reaction is observed with CH 3Cl, while CH 3Br undergoes a slow reaction (reaction efficiency 1.2%) and CH 3I a fast reaction (reaction efficiency 21.6%) with 1 +. In both reaction systems, the main product ion C 6H 5As +X is produced by radical substitution with transfer of the halogen atom X to 1 +. The methylphenylarsenium ion C 6H 5As +CH 3 is found as a second product ion in both systems but with distinctly less intensity. Unexpectedly, an ion C 7H 7 + is observed as a third product only of the reaction system 1 +/CH 3Br. Information about reaction enthalpies was obtained by theoretical studies which show that the reaction with CH 3Cl would be only slightly exothermic, if at all. Further, generation of the less abundant product C 6H 5As +CH 3 is the more exothermic pathway for CH 3Br and CH 3I. This proves that reaction mechanisms and not reaction exothermicity determines the total rate constant and the branching ratio of the ion/molecule reactions observed. It is suggested that the mechanism for generation of G 6H 5As +X is electrophilically assisted radical substitution via formation of an intermediate adduct of electrophilic 1 + to an electron lone pair of the halogen of CH 3X. In contrast, C 6H5As +CH 3 is generated via insertion of the carbenoid 1 + into the C X bond which yields a halogenomethylphenylarsane radical cation C 6H 5As(X)CH 3 + as intermediate. According to the calculations, this insertion is strongly exothermic but requires obviously a substantial activation energy. The investigation of the fragmentations of C 6H 5As(X)CH 3 by tandem mass spectrometry corroborates these suggestions. Notably, only C 6H 5As(Br)CH 3 +, but not C 6H 5As(I)CH 3 +, generates abundant C 7H 7 +ions on fragmentation.