This invention relates to a method of preparing hydrazones of the formula ##EQU1## in which R.sub.1 and R.sub.2, which may be identical or different, represent a linear alkyl radical of from 1 to 12 carbon atoms, a branched alkyl or cycloalkyl radical of from 3 to 12 carbon atoms, an aromatic radical of from 6 to 12 carbon atoms having a benzene or naphthalene nucleus, or a radical of from 5 to 12 carbon atoms having a pyridine nucleus, or R.sub.1 and R.sub.2 together form a linear or branched alkylene radical of from 3 to 12 carbon atoms in which one of the carbon atoms of the chain may be replaced by an oxygen atom, all of the above radicals being unsubstituted or substituted by one or more atoms or groups such as chlorine, bromine, fluorine, or iodine atoms or hydroxy, ether, carboxylic acid, carboxylic amide or ester, nitrile, nitro, or sulphonic acid or amide groups, or one of the two radicals R.sub.1 and R.sub.2 may be a hydrogen atom, and in which R.sub.3 and R.sub.4, which may be identical or different, represent a hydrogen atom, a linear alkyl or cycloalkyl radical of from 3 to 12 carbon atoms, in which one of the carbon atoms of the cycloalkyl radical may be replaced by an oxygen atom, or an aromatic hydrocarbon radical of from 6 to 12 carbon atoms or R.sub.3 and R.sub.4 together represent a linear or branched alkylene radical of from 3 to 11 carbon atoms, the above radicals being unsubstituted or substituted by atoms or groups such as chlorine, bromine, fluorine, ethylenic groups, nitro, hydroxy, alkoxy, or carboxylic acid or ester radicals , which comprises reacting a primary or secondary amine of the formulaWherein R.sub.1 and R.sub.2 are as defined above, ammonia, and a carbonyl compound of the formulaWherein R.sub.3 and R.sub.4 are as defined above, with a peroxide compound.