The interaction between two kinds of photographic gelatin and chemical sensitizers was studied by X-ray Photoelectron Spectroscopy. After the gelatin samples were immersed into the chloroauric acid solution for 5 min, methionine and methionine sulfoxide in gelatins were oxidized to methionine sulfone, reducing most parts of Au³⁺ to Au which was coordinated by gelatin macromolecule. According to the ratio of Au³⁺ to Au in gelatins, it is shown that the reducing power of Rousselot gelatin is stronger than that of Baotou gelatin. When sodium thiosulfate was added to gelatins, methionine sulfoxide was reduced to methionine. Sodium thiosulfate, which is a reductant stronger than both methionine and methionine sulfone, could coexist steadily with methionine and methionine sulfone in the gelatin. Two kinds of photographic gelatins added with sodium thiosulfate could reduce Au³⁺ to colloid gold. In this redox reaction, the main reductant is sodium thiosulfate, but not methionine in the gelatin. The addition of sodium thiosulfate enhanced the reducing power of the photographic gelatin.