The ENDOR technique has been used to study ¹³³Cs hyperfine couplings of SO₃^– radical ions in X-irradiated Cs₂S₂O₆ single crystals. The orientational dependent parameters for two couplings were obtained from the ENDOR spectra and assigned with the aid of ENDOR-induced EPR. The larger coupling, a(I)= 30.55 MHz, was attributed to the Cs⁺(2)S(2)O₃˙^– interaction and the smaller coupling, a(II)= 9.59 MHz, to the crystallographically different product Cs⁺(2)S(1)O₃˙^–. The difference in distances was estimated from the experimentally measured hf splittings using a point-dipole approximation, and was found to be in agreement with crystallographic data. A temperature dependence of hyperfine couplings was observed and possible explanations are discussed in terms of the displacements induced by the piezoelectric and pyroelectric properties of the Cs₂S₂O₆ crystal.