The four dihalogeno phenylarsanes C 6H 5AsF 2, 1, C 6H 5AsCl 2, 2, C 6H 5AsBr 2, 3, and C 6H 5AsI 2, 4, produce in the 70 eV-EI mass spectra by loss of a halogen atom abundant halogeno phenylarseniun ions C 6H 5As +–X, 1a +– 4a +. The further fragmentation reactions of ions 1a +– 4a + are elimination of a molecule halogen hydride HX and/or loss of a halogen atom X . The preferred route of fragmentation depends clearly on the strength of the As–X bond. The metastable fluoro ion 1a + and chloro ion 2a +, respectively, fragment only by loss of HF and HCl, the metastable bromo derivative 3a + exhibits losses of HBr and Br of about equal intensity, and the metastable iodo ion 4a + fragments only by loss of an I atom. The loss of HX is associated with a large kinetic energy release (KER) which yield a dish-topped peak in the MIKE spectrum of 1a + (〈 T〉 = 845 meV) and 2a + (〈 T〉 = 550 meV) and a broad round-topped peak on the MIKE spectrum of 3a + (〈 T〉 = 369 meV). Theoretical calculations (UBHLYP/6-311 + G(2d,p)/-/UBHLYP/6-31 + G(d)) confirm that the elimination of HX requires an enthalpy of activation Δ H #, and that in the case of 3a + this Δ H # and the reaction enthalpy for loss of Br are of similar size. The Δ H # of HX elimination is also responsible for the reverse enthalpy of activation Δ H rev # and the KER during this process. The observed 〈 T〉 amounts to 78% ( 1a +), 55% ( 2a +), and 40% ( 3a +) of the calculated value of Δ H rev # . Thus, the KER as well as the distribution of the KER (KERD) and the competition between elimination of HX and loss of X are very specific for the halogen ligand at the As atom. Metastable (4-fluorophenyl) arsenium ions 1b +, F–C 6H 4As +–H, and (4-chlorophenyl) arsenium ions 2b +, Cl–C 6H 4As +–H +, eliminate HF or HCl with virtually identical KERD as metastable 1a + or 2a +, C 6H 5As +–Cl, proving an identical transition state for both isomers. Accordingly, 1b + and 2b + rearrange to 1a + and 2a + prior to HCl elimination. Such a rearrangement by a reductive elimination/oxidative insertion of As + into C H and C X bonds has been observed before and appears to be typical of arylarsane radical cation and arylarsenium cations.