The electron spin resonance (ESR) spectra of γ-irradiated potassium pyrosulfite single crystals have been investigated between 300 and 103 K temperature interval. The signals were attributed to three SO-3 and a SO-2 radicals, and found to be temperature dependent. The SO-3 radicals were found to be inequivalent. The presence of three SO-3 radicals is not consistent with the monoclinic structure of K2S2O5 single crystal and this is attributed to any misadjustment of the orientations of SO-3 radicals to fit the crystal structure after irradiation. Elements of the g and hyperfine tensors of 33S nuclei with natural abundance of 0.74% for SO-3 and SO-2 radicals were determined. For two SO-3 radicals, the g and hyperfine values in a plane perpendicular to a axis of the crystal were found to be equal below 193 K, while they are different at room temperature. This reversible effect can be attributed to structural phase transition. © 1992.