In analogy to the authors' successful PX2+ insertion reactions, an AsX2+ insertion route was explored to obtain new arsenic halogen cations. Two new salts were prepd.: AsBr4+[Al(OR)4]-, starting from AsBr3, Br2 and Ag[Al(OR)4], and I2As-PI3+[Al(OR)]4 from AsI3, PI3 and Ag[Al(OR)4] (R = C(CF3)3). The first cation is formally a product of an AsBr2+ insertion into the Br2 mol. and the latter clearly a PI2+ insertion into the As-I bond of the AsI3 mol. Both compds. were characterized by IR and NMR spectroscopy, the first also by its x-ray structure. Reactions of Ag[Al(OR)4] with AsI3 do not lead to ionization and AgI formation but rather lead to a marginally stable Ag(AsI3)2+[Al(OR)]4 salt. Despite many attempts the authors failed to prep. other PX-cation analogs such as AsI4+, As2X5+ and P4AsX2+ (X = Br, I). To explain these neg. results, the thermodn. of the formation of EX2+, EX4+ and E2X5+ (E = As, P; X = Br, I) was carefully analyzed with MP2/TZVPP calcns. and inclusion of entropy and solvation effects. As2Br5+ is in very rapid equil. with AsBr2+ and AsBr3 (DG Deg(CH2Cl2) = +30 kJ mol-1). The extremely reactive AsBr2+ cation available in the equil. accounts for the obsd. decompn. of the [Al(OR)4]- anion. By contrast, the stability of AsI3 against Ag[Al(OR)4] appears to be kinetic and, if prepd. by a suitable route, As2I5+ would be expected to have a stability intermediate between the known P2I5+ and P2Br5+. [on SciFinder (R)]