The invention relates to a process and a nucleophilic reagent which is useful for grafting a substituted difluoromethyl group onto a compound containing at least one electrophilic function, characterized in that it comprises: a) a fluorocarboxylic acid of formula Ew--CF.sub.2 --COOH where Ew represents an electron-withdrawing atom or group, at least partially salified with an organic or inorganic cation, and b) a polar aprotic solvent; and in that the content of releasable protons carried by its various components, including their impurities, is at most equal to half the initial molar concentration of the said fluorocarboxylic acid.