A photographic hardening agent consists of a polymeric compound having a linear chain to which are attached, directly or indirectly, at least 3% of a carboxyl group or groups derived from a ,b -unsaturated dicarboxylic anhydrides, and attached indirectly at least 2% of unsaturated hydrocarbon terminal aldehyde, maleimide or mucochloryl groups. The polymer may be a copolymer of ethylene and N-aldehydomethyl maleamic acid, b -maleimidoethyl acid maleate, b -vinylsulphonylethyl acid maleate or mucochloryl acid maleate, of ethylenemaleic acid and maleimidomethyl acid maleate, of acrylic acid and b -maleimidoethylacrylate, of vinyl alcohol, vinyl acrylate, vinyl acetate and vinyl acid succinate of N-acrylyl - o - mucochloryl carbamate and maleic anhydride. The polymer may also be b -acrylyloxyethyl carbamyl gelatin or acrylyl carbamyl gelatin. The polymer, e.g. 5-10%, may be compounded with gelatin. In examples, the polymers were prepared by treating (1) an ethylene/maleic anhydride copolymer with amino - acetaldehyde diethyl acetal, an ion-exchange resin, and sodium hydroxide; (2) an ethylene/maleic anhydride copolymer with N - hydroxymethyl - maleimide; (3) an ethylene/maleic anhydride copolymer with b -hydroxymethylmaleimide, hexamethylphosphoramide and pyridine; (4) polyacrylic anhydride as in (3); (5) polyvinyl alcohol with acrylyl chloride, pyridine and succinic anhydride; (6) an ethylene/maleic anhydride copolymer with vinyl hydroxyethyl sulphone and pyridine; (7) an ethylene/maleic anhydride copolymer with mucochloric acid; (8) gelatin with dimethyl sulphoxide and either isocyanatoethyl acrylate or acrylyl isocyanate. In a further example, maleic anhydride and N-acrylyl - 0 - mucochloryl carbamate were copolymerized in benzene using benzoyl peroxide as catalyst.