Intramolecular sulphur–iodine (S∴I–)⁺ and sulphur–bromine (S∴Br–)⁺ three-electron-bonded radical cations have been observed during the oxidation of various iodo-and bromo-alkylthioalkanes by ·OH radicals. For the bromo compounds, in addition, intermolecular (S∴S)⁺ radical cations are stabilized at high solute concentrations. Oxidation of alkylthio (chloro) alkanes leads only to the dimer sulphur-sulphur three-electron-bonded radical cations. All these radical cations exhibit optical absorptions in the visible and near u.v. with extinction coefficients in the order of 4 000–8 000 mol^–1 dm³ cm^–1. In general, increasing the chain length between the sulphur and the halogen atoms, and increasing the electron release by the sulphur substituent, results in a red shift of the λ_max. value and a decrease in radical cation yields. In aqueous solutions most of the three-electron-bonded species decay by first-order kinetics with t_1/2 up to 110 µs.