Five kinds of gelatins were investigated, and varying amount of sodium thiosulfate was added to study the effect of Na₂S₂O₃ on photographic gelatin. The ubiquinone and RS· radicals were detected and the change of their concentrations in photographic gelatins were studied by electron spin resonance (ESR). The concentration of ubiquinone, which was dependent on the equilibrium among the hydroubiquinone, ubiquinone radical and ubiquinone compounds, increased to a maximum value and then decreased with the increasing amount of sodium thiosulfate. The study of the chemical state of iron in gelatin with varying amounts of sodium thiosulfate by x-ray photoelectron spectroscopy (XPS) showed that the state of iron in gelatin was not affected by adding sodium thiosulfate.