The molar absorption coefficient (ε) of I₃⁻ at λ_max, and the equilibrium constant (K_eq) for the equilibrium I₂ + I⁻ ⇌ I₃⁻ were determined for various alcohol/water mixtures by using standard analytical procedures, i.e. measurement of optical densities via UV spectrophotometry. The stability of I₃⁻ is found to increase significantly with increasing amount of alcohol with K_eq ranging from 7.1 × 10² dm³ mol⁻¹ in H₂O to, e.g., 2.5 × 10⁴ dm³ mol⁻¹ in 70% (v/v) of 2-propanol/H₂O mixture. Also, by increasing the amount of alcohol, the position of λ_max for I₃⁻ is shifted to longer wavelengths (from 350 to 357 nm). This red-shift is accompanied by a drop in ε from 25 400 dm³ mol⁻¹ cm⁻¹ in pure H₂O to, e.g., 14 000 dm³ mol⁻¹ cm⁻¹ in a 70% (v/v) of a 2-propanol/H₂O mixture. The obtained data were applied for quantitative determination of I₃⁻ by means of the time-resolved technique of pulse radiolysis, where this product was formed via oxidation of a total of three iodide ions by each CCl₃OO˙. The overall process afforded formation of one equivalent each of I₂ (complexed by I⁻ to I₃⁻) and I˙ (complexed to I₂˙⁻ and suffering as such disproportionation). An excellent agreement was obtained with the data from the standard analytical measurements. Furthermore, the radiation chemical yields of I₃⁻ generated in these pulse radiolysis experiments (e.g., G(I₃⁻) = 0.96 μmol J⁻¹ in 2-propanol/H₂O mixtures) are in complete correspondence with a previously suggested multi-electron oxidation mechanism. Complementary steady-state product analysis of γ-irradiated solutions confirmed the formation of I₃⁻ in high yields, with limiting values of G ≅ 0.9 μmol J⁻¹ in 2-propanol/water mixtures over wide ranges of pH, iodide, CCl₄, CH₂Cl₂ and 2-propanol concentrations. All the results also strongly corroborate corresponding oxidation mechanisms of organic sulfides and selenides by halogenated peroxyl radicals.