A method of hardening gelatin comprising adding to the gelatin to be hardened at least one compound represented by formula (I) ##STR1## wherein L represents a divalent organic group, R represents a hydrogen atom or a univalent organic group, A represents a substituted or unsubstituted benzene ring which may be condensed with another ring, M.sup..sym. represents a hydrogen ion, an alkali metal ion or an ammonium ion, and n has a value of 1 or 2 or 3. The method may be applied to various processes involving hardening of gelatin, such as processes for forming various photographic materials or microcapsules.