The transient optical absorption spectrum produced on pulse radiolysis of an N₂O saturated aqueous solution (pH 3.0–10.0) of thiodiglycolic acid has been assigned to the α-thio radical, HO₂CH₂C–S–ĊHCO₂H, formed by hydrogen atom abstraction from the parent compound by hydroxyl radicals. The absorption maximum (285 nm) and extinction coefficient (1.6 × 10³ dm³ mol^–1 cm^–1) are found to be independent of solute concentration over the range 8.0 × 10^–5-8.0 × 10^–2 mol dm^–3 and pH (1.0–10.0) of the solution. The rate constant for the reaction of hydroxyl radicals with thiodiglycolic acid has been determined to be 4.1 × 10⁹ dm³ mol^–1 s^–1 and the product species decays by second-order kinetics with a rate constant of 1.6 × 10⁹ dm³ mol^–1 s^–1 Strong oxidizing agents such as (CH₃I)₂⁺˙, Cl₂^–˙, CH₃İOH, and SO₄^–˙ have been found to oxidize thiodiglycolic acid with the formation of a transient species with λ_max 280 nm, the rate constants being 6.7 × 10⁹, 4.9 × 10⁸, 6.9 × 10⁹, and 2.7 × 10⁹ dm³ mol^–1s^–1 respectively. The transient optical absorption spectrum (λ_max= 280, 380 nm) obtained on pulse radiolysis of a nitrogen saturated solution of thiodiglycolic acid in 1,2-dichloroethane has been assigned to a radical cation derived from this solute.