In order to investigate causes of the oxidative deterioration, photographic gelatins were oxidized with 0.1-5% hydrogen peroxide, and physical and chemical properties of oxidized gelatins were studied. Viscosity, jelly strength, gelling time decreased with an increase of the extent of oxidation. Optical rotatory properties also changed. Chemical analyses of oxidized gelatins showed that oxidation caused the decrease of high molecular weight fractions (higher than 2×10⁵ dalton) with an increase of low molecular weight fractions (lower than 1×10⁵). Decomposition of amino acids and cross-linked amino acids included in gelatin molecules was also observed; especially glutamic acid, methionine, tyrosine, and lysinoalanine residues decreased as oxidation proceedcd. In spite of these changes, composition and quantities of N-terminal amino acids little changed after oxidation. Thus, we concluded that oxidation degrades gelatin molecules by oxidative cleavage of peptide bonds and cross-linkages to result in changes of physical properties of photographic gelatins. The cleavage seemed to accompany the masking of amino groups of cleaved peptide bonds. Destruction of amino acids also may be a cause of the deterioration of gelatins.