Solid sodium thiosulfate, anhydrous and hydrated (5 H₂O), was irradiated with 10 MeV electrons in a pulsed, high-dose (30 kGy) rate regime. The resulting optical absorption was measured by diffuse reflection spectophotometry (DRS) (absorption band in anhydrous salt at 360 nm, in hydrated salt at 418 nm). Absorption decays occurred at t_1/2 = 80 h (anhydrous) and t_1/2 = 9 h (hydrated) at room temperature. The yellowish radical ion formed in irradiated anhydrous thiosulfate was identified as [^·S₂O₃]^-, and not [^·S₂O₃]^3-, as proposed in previous nonoptical (EPR) investigations. The absorption spectrum of [^·S₂O₃]^- in a solid matrix was similar to the transient spectrum ascribed to the same radical, obtained by pulse radiolysis in aqueous solution, and had a lifetime many orders of magnitude shorter. A similar radical anion, but in the complex with adjacent water molecules from the hydration moiety, was observed in hydrated thiosulfate (Na₂S₂O₃·5 H₂O).