A process for hardening gelatin which comprises reacting photographic gelatin with a compound of the general formula ##SPC1##Wherein R.sub.1 and R.sub.2 individually are halogen, an aryloxy group which may have substituents, or an arylthio group which may have substituents; X is a simple bond, an alkylene group which may have substituents --CO, --CO--R.sub.5 --CO-- (in which R.sub.5 is an alkylene group which may have substituents) or --R.sub.6 --(O--R.sub.6).sub.n -- (in which R.sub.6 is an alkylene group which may have substituents and n is a positive integer); an Y is a saturated or unsaturated hydrocarbon group which may have substituents, or a group ##SPC2##(in which R.sub.3 and R.sub.4 individually are halogen).