The structural origin of reversible gamma-induced effects in vitreous Ge23.5Sb11.8S64.7 has been investigated by high-resolution X-ray photoelectron spectroscopy (XPS). The changes in valence band spectrum from gamma-irradiation suggest a decrease of sulfur lone pair electron concentration accompanied by changes in bonding states of S and Ge. The appearance of additional doublets in the core-level XPS spectra of Ge, Sb, and S atoms for gamma-irradiated sample is described by the formation of over- and under-coordinated charged defect pairs (Ge-3(-)-S-3(+)) as a result of radiation treatment. The results verify the switching of Ge-S covalent bonds into S-S bonds as the main microstructural mechanism for gamma-induced optical effects in this glass.