Abstract: The interaction between doped ferric ion and photographic gelatin was investigated by XPS. ESR. The results show that gelatin has very strong reducing power for the added ferricion, but the reducing capacity is different for different gelatins. When the amount of the added ferric ions exceeds this capacity, some ferric ions will not be reduce and remain their original trivalence. When the added ferric ions are reduced, their chemical state in gelatin is different from that of the ferrum originally existing in gelatin. The density of outmost shell electron cloud of the former is less than that of the latter, i.e. the ligands that form complex with added ferrum are different from that with original ferrum. The reduction capacity is very high compared with the admitted amount of ferrum in inert gelatin and varies in different gelatins. After reduced,the ferrem was coordinated by gelatin macromolecule and the coordination environment is different from the coordination environment of ferrum in undoped gelatin. The doped ferric ion not only effect the spin concentration of Ubiquinone in gelatin, but also the production and disappearance of RS. free radical （g=2.246, g= 1.934）. In gelatin E the addition of ferric ions may cause monoelectron transfer of the ubiquinone radical.